1. In a large pot, saute in olive oil the onion, garlic and basil. Add tomatoes, crushing them with your hands as they are added to the pot. Mix well. Transfer mixture to food processor, pureeing to a chunky consistency.
2. In a large saucepan begin heating milk, being careful not to boil. In a frying pan, melt butter and slowly add flour, mixing with a wire whip, about 5 minutes. Add butter/flour mixture to milk, stirring well as sauce thickens, about 8-10 minutes. Add this mixture to tomato mixture and heat to holding temperature. Season with salt and pepper to taste. Garnish bowls with a sprig of basil.
Makes approx. 1 gallon, or 15 - 8 oz. servings.