This is an easy sauce to make with a bumper crop of ripe summer tomatoes. Unlike store-bought sauces from a jar, it has a fresh, vibrant taste. It will keep in the refrigerator for one week, or it can be frozen for 3 months. It works well as a base for a chicken, eggplant parmigiana, or spaghetti bolognese.
Position a rack in the middle of the oven and preheat to 475 degrees F.
Wash tomatoes and remove stems. Cut into quarters (or eighths if they're very large) and transfer to a large bowl. Peel the onion and cut into 1-inch wedges and add to the bowl, along with the garlic cloves. Drizzle the oil over the mixture and toss to coat. Transfer to a broiler pan or shallow baking pan and cook for 10 minutes at 475 degrees F. Then, reduce heat to 425 degrees F and continue cooking for 40 to 50 minutes, or until the onions are very soft. Let mixture cool completely.
When tomatoes are cool, remove skins and discard. Squeeze the garlic cloves and discard skins. Transfer the sauce to a blender or food processor and purée in batches.
Place sauce in a medium pan and add the basil, parsley, red pepper flakes, salt, and sugar to taste. Warm slowly over medium heat until sauce is hot. Serve immediately.