Core tomatoes and cut into large chunks. Place in a large pot with the basil sprigs. Peel the onion and cut in half. Put half the onion away. Cut the other half in two or three large wedges and add to the tomatoes. Place over medium heat and bring to a simmer. Cook, stirring occasionally, until the tomatoes are broken down. This should take 1 ½ or 2 hours.
Carefully pour the sauce into a food mill, or if you do not have one into a mesh strainer, over a large bowl. If using the food mill, pass all of the sauce through and discard the solids. If using a strainer, use a spatula or spoon to press on the mixture until it is all passed through, except for the skin and seeds and discard them.
At this point you can refrigerate or freeze the unseasoned tomato sauce or portions of it.
Chop the remaining onion finely and finely chop the garlic. Warm the olive oil in a large pot and saute the onions and garlic until softened, about 8-10 minutes. Pour in the sauce plus the sugar and salt and stir. Bring to a simmer and place a lid slightly ajar to catch any spatters. Cook for 35-45 minutes until thickened. Taste for seasoning and adjust salt if necessary.
Sauce can be frozen up to a month. Use on pasta, in eggplant parm, with meatballs, or as a pizza sauce.
I have two favorite ways to utilize this sauce, and both involve eggplant! A perfect companion for ripe summer tomatoes.
Pasta alla Norma - In a deep wide skillet, warm 3-4 tablespoons extra virgin olive oil. Toss in cubed eggplant and cook over medium heat without stirring, for 2-3 minutes. Stir and cook another 2-3 minutes without disturbing. Pour in a few cups of tomato sauce and simmer partially covered until the eggplant is perfectly tender. Taste for seasoning, and toss with cooked penne pasta. Top with fresh basil and ricotta salata or fresh parmigiano.
Grilled Eggplant Parm - Slice 2 or 3 medium to large eggplant into thick slices (about ¾ inch thick). Brush with olive oil and sprinkle with salt and grill until softened and cooked through. Place a few spoonfuls of tomato sauce in the bottom of a baking dish, top with the grilled eggplant, more sauce, layering if necessary to fit. Top with sliced fresh mozzarella and a large handful of freshly grated parmigiano. Bake at 350 until warmed through and cheese is melted and bubbly, about 15-20 minutes. Sprinkle with freshly chopped basil.