Place the olive oil, 1 tablespoon of butter, and onions in a large saucepan set over medium-low heat. Cook the onions, stirring often, until tender and translucent, about 5 minutes. Add the garlic and cook ofr an additional 30 seconds.
1 tablespoon olive oil, 6 tablespoons unsalted butter, 1 medium yellow onion, 2 cloves garlic
While the onions cook, pull or cut off the tough stem end of each tomato and crush the tomatoes with a potato masher. Pour all the tomatoes and their juices into the pan with the onion/garlic mixture.
56 ounces Certified San Marzano whole peeled tomatoes
Add the bay leaf, basil, remaining 5 tablespoons of butter, kosher salt, black pepper, and red pepper flakes. Stir well.
1 bay leaf, 2 sprigs fresh basil leaves, ¾ teaspoon kosher salt, ¼ teaspoon ground black pepper, 2 pinches crushed red pepper flakes
Bring the sauce to a simmer, reduce heat to low, and cook until sauce has thickened, about 45 minutes. Discard the bay leaves and basil stems. If desired, puree the sauce with a stick blender or in a stand blender.