Place all ingredients in a pot. Simmer until sauce is thickened, 10 minutes. If necessary, place sauce in a food processor and pulse until smooth.
For grilled tomato sauce, rub tomatoes with oil, then grill until charred. Puree tomatoes and add to Sauteed onions. Season with salt and pepper.
For roasted tomato sauce, halve tomatoes and roast, coated with oil and sprinkled with salt and pepper on a sheet pan in 375 degree oven for 45 minutes. Puree in a blender and season with fresh basil and salt and pepper.
VARIATIONS: Add 3 tablespoons sundried tomatoes, minced Stir in 1/3 cup pesto. Stir in 1/2 cup pitted black kalamata olives.