Lightly oil a large, rimmed baking sheet and heat the oven to 325 F.
Slice or chop the tomatoes into 1-inch to 2-inch pieces. Discard stems and any tough cores.
In a large bowl, toss the chopped tomatoes, shallots, and garlic with the Italian seasoning, salt, pepper, red pepper flakes, and olive oil.
Spread the tomato mixture out on the baking sheet and roast for 1 hour.
Put the tomatoes and vegetables through a food mill—this will eliminate seeds and peels. Alternatively, put the tomato mixture in a blender and blend until smooth.
Transfer the tomato mixture to a saucepan and add the tomato paste.
Place the saucepan over medium heat and bring the sauce to a simmer. Reduce the heat to low and simmer, uncovered until thickened as desired, about 10 minutes. Add the basil and continue cooking for 4 to 5 minutes.
Taste the sauce and adjust the seasonings. Add a little sugar if a sweeter sauce is desired.
Use the sauce immediately or cool it and transfer it to an airtight container.
Refrigerate the sauce and use within 3 to 4 days or freeze it for longer storage.