In a large pot, heat the oil over medium heat until hot and rippling. Add the onion and garlic and saute, stirring very frequently, until the onion is soft and translucent, about 3-4 minutes. Add the celery and carrots and season with salt and pepper. Cook, stirring frequently, until all the vegetables are slightly tender, about 5 minutes. Add the crushed tomatoes, basil, oregano and bay leaves and simmer, covered, on low heat for 1 hour, stirring occasionally to make sure the sauce isn’t sticking to the bottom of the pot, until slightly thickened and the vegetables are completely tender. Stir in the brown sugar.
Taste the sauce. Add salt and pepper as needed. Add the butter and stir to combine if you want to round out the flavors and add a dimension of silkiness.
Remove the bay leaves. In a blender or food processor, blend the sauce (in several batches, if necessary) until it is smooth. The sauce can be frozen once it is cooled completely in freezer-safe ziploc bags for up to 6 months.