Drizzle olive oil in the bottom of the largest stock pot or Dutch oven that you have. This recipe is going to take some space! Heat oil over medium-low heat. Add butter to pan and allow it to melt in the oil. Add red onions, salt and pepper them to taste, and allow them to caramelize in the oil and butter. This takes a little while, you want the onions to loose some of their bite and taste sweet. If they are sizzling or browning, they are cooking too fast, turn them down and let them cook, stirring occasionally for 35-45 minutes. Don't panic - this part takes as long as the rest of the recipe put together.

Once onions are caramelized, raise heat to medium. Add garlic and let it cook with the onions for about a minute, or until the garlic becomes fragrant. Add previously roasted tomatoes, with their skins removed. I pull them apart with my fingers as I add them to the pot. You can certainly substitute organic tomatoes from a BPA-free can here - but fresh tomatoes that have been roasted in the oven are so sweet, once you try it you will never want them out of a can again. Stir to combine.

Add finely chopped, fresh basil. Don't skimp on the basil. The fresh flavor makes a world of difference in the end result. Add dried oregano - I actually don't mind dried oregano over fresh because oregano can be pungent when fresh. If you have fresh oregano and prefer it, I would start with 4 tablespoons of fresh and add 1-2 tablespoons more by taste. (the general rule of dried versus fresh herbs is 1:3. I prefer to be a little conservative with oregano so it does not become overpowering.) Add red pepper flakes. Salt and pepper the sauce to taste.

Allow it to cook for 20 minutes at a low simmer, stirring occasionally.

Use your immersion blender to puree the sauce to your desired consistency. I like mine to be blended but still chunky. Because the tomatoes have been roasted previously, much of the water will have cooked out of them so your sauce will not have to reduce. It will be thick. YUM! Make sure to taste your sauce and adjust seasonings based on your preference.

If you do not have an immersion blender, you can do this in batches in your blender or food processor. Be careful not to burn yourself. By the time you get through all the sauce, you might decide that a $25 immersion blender is a good investment. :)

Allow the sauce to cool and freeze it in freezer bags or freezer-safe glass containers. If you choose to freeze in glass, make sure you leave the appropriate amount of head space in your jars. Food expands when cooking, which can cause glass to break. If you freeze in freezer bags, freeze them laying flat. Once frozen, you can stand them up and it will take much less room in your freezer. If you are interested in canning your sauce for the pantry (because there is only so much room in the freezer!), make sure you check with your local Extension Service or the National Center for Home Food Preservation for advice on waterbath versus pressure canning tomato based recipes.