Serve this sauce over cooked pasta; top with freshly grated Parmesan.
Place tomatoes in a colander and sprinkle with 1 teaspoon salt. Let liquid drain into sink for about 30 minutes.
Heat oil in a 12-inch frying pan. Add garlic and red-pepper flakes, if desired, and cook until garlic begins to color. Add onion and cook over low heat until very soft but not brown, about 10 minutes.
Add tomatoes and basil. Add salt, pepper, and sugar. Turn heat to medium high and cook 5 to 10 minutes, until thick (sauce should bubble briskly). Stir in parsley and adjust seasonings.