Will Onions Soften Or Dissolve In Tomato Sauce?

A soft and smooth tomato sauce is a delight for many Italian and other Mediterranean dishes. Onion is often added to add flavor and depth to the sauce but the diced-up pieces can get in the way. Getting a smoother sauce has a few steps to follow.

If you’re reading this, you might have even forgotten to add your onion to your sauce and are wondering if you can add diced raw onion to soften up later on.

Onions will soften slightly but won’t dissolve in tomato sauce. If you add them later on in the cooking process, they might remain raw and crunchy. The best way to soften onions is to sauté them before simmering.

If you forget to add them at the beginning of cooking, it’s best to fry them in a separate pan and then add to your sauce. Or you can place half a raw onion into the simmering sauce to infuse flavor and remove it.

Let’s look at the best way to prepare your onion to make a tomato sauce. I’ve included a recipe and some of my tips to get a perfectly smooth sauce.

Should You Cook Onions Before Putting In Sauce?

It is always best to cook onions (and meat) before adding them to tomato sauce. This is because raw onions can have a sharp, harsh flavor that takes some direct heat to mellow out, become sweet, and soften.

However, if you add them later on, they will not likely cook off and soften as simmering them in a sauce is not as quick or effective as frying directly in a pan. They will also still have that raw taste that could overpower the other flavors in your sauce.

Wondering what onion is best to use? Check out my post on using red onion vs white onion for pasta sauce.

How To Soften Onions For Pasta Sauce

There are a few ways you can cook onions before adding them to your sauce. The most common way is to sauté them in a pan with some oil until they are softened and translucent.

You can also fry them a bit longer in the oil until they are golden brown and caramelized – see my tips on caramelizing onions. This will give your sauce a deeper, sweeter flavor. Roasting them is another interesting method to boost flavor.

Once the onions are cooked, add your other ingredients and canned tomatoes or sauce and continue with the recipe as normal.

Basic Recipe For Tomato Sauce

A good tomato sauce can be used as a base for so many dishes including many types of pasta and pizza so it’s always good to have a basic recipe handy.

This quick tomato sauce is simple but packed full of flavor. Buy the best quality canned tomatoes as they are the key ingredient. Serves 4 people. Takes about 30 minutes in total.


  • 2 tablespoons of olive oil
  • 2 cloves of garlic
  • 1 small onion
  • 28oz (2x 400g) can of whole or crushed tomatoes
  • 1 teaspoon dried oregano
  • Salt and pepper

The steps are pretty much the same for most basic tomato sauce recipes:

  1. Dice the onion and mince the garlic
  2. Cook the onions over medium heat with the oil until softened
  3. Add the garlic and cook for a few minutes until it smells fragrant
  4. Crush the tomatoes with your hands or use a potato masher and add to the pan
  5. Stir in the herbs and simmer for around 20 minutes until the sauce has reached the desired consistency
  6. Season with salt and pepper
  7. Blend it if you want a smooth sauce

This will give you a lovely tangy sauce that is really versatile. You can even make it in larger batches and freeze portions of it once cooled.

Will Raw Onions Cook In Pasta Sauce?

Raw onions will not cook or soften that much when simmering in a tomato sauce. They might still have a crunchy texture and their flavor will not be as mellow or sweet.

As the simmering is not such direct heat as frying, it takes ages for the onion to cook this way.

If you are looking for a quick fix, just saute them for 5 minutes in some oil first to take off the rawness then add them to your sauce.

Or just place half an onion in the sauce directly and remove it later – it will flavor the sauce as it simmers.

Another option is to grate the onion instead of chopping it. This will help it cook a bit faster as the pieces are smaller, although the raw flavor will probably still be there if you leave it too late to add them.

Will Onion Dissolve In A Pasta Sauce?

An onion will not dissolve in pasta sauce no matter how long you cook it. Grating the onion will help, but to get a completely smooth sauce you must use a blender or food processor.

Onion does not dissolve but it does take on a softer consistency as it cooks and generally just becomes a part of the sauce.

However, no matter how fine you chop up your onion, what method you use to cook it before adding it to the sauce or how long you leave it, those small bits of onion will still be there. 

There are a couple of ways to get rid of them or at least make them less noticeable, which I’ll take a look at next.

How To Create A Smooth Sauce

One way to get a smoother tomato sauce with fewer noticeable pieces of onion is to grate it beforehand so that the pieces are very small and barely noticeable to most people in the finished product.

If you want to create a smooth sauce though with no lumps at all, the best way to achieve this is to blend it after cooking. This will get rid of any lumps or bits of onion (or anything else for that matter) and give you a nice smooth sauce.

You can use an immersion hand blender like this one from Amazon, a food processor, or even just a regular blender.

Personally, I prefer to use an immersion hand blender as it means I can finish off everything in the same pan.

How to Thicken Tomato Sauce

Sometimes you may find that your tomato sauce refuses to thicken up and unless you have the time to leave it simmering for 20-30 minutes (always with the lid off of course) then there is another method that works much quicker.

Mix up a thickening slurry using 2 tablespoons of cold water with a tablespoon of cornstarch. Whisk this until it’s combined and then gradually pour it into your simmering tomato sauce a small amount at a time.

The heat will thicken the starch over the next few minutes as it simmers. Mix it in as you go and stop when your sauce has your preferred consistency.


So, there you have it. Will onions soften in tomato sauce? Not really. Will onion dissolve in a pasta sauce? Definitely not!

If you can, cook the onion beforehand for the best results but if not, use one of my tips to get around adding it later and still end up with a delicious and satisfying final product. 

Make the basic tomato sauce recipe for a quick and easy sauce that you can use for lots of dishes and remember that if you are after something smooth without any lumps then you can always use a blender of some sort right at the end to finish it off.

Have any questions? Ask me in the comment section below and I’ll get back to you.

Tom Hambly

Tom Hambly is the founder of Boss The Kitchen. With a background in cooking and building websites, he enjoys running this site to help other cooks improve. About Tom Hambly.

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