In a large saucepan cook onion in hot oil over medium heat about 8 minutes or until golden brown, stirring frequently.
Stir in remaining ingredients. Bring to boiling; reduce heat. Simmer, uncovered, for 30 to 40 minutes or until sauce is thickened, stirring occasionally. Remove from heat; cool sauce.
Transfer sauce, half at a time, to a blender container or food processor bowl. Cover and blend or process sauce until smooth. Store, tightly covered, in the refrigerator up to 1 month. Makes 3-1/2 cups sauce.
25 or 50 SERVINGS:
For 25 servings, double all ingredients. For 50 servings, quadruple the ingredients. For either amount, cook onions as directed in a 6-quart Dutch oven. Stir in remaining ingredients and simmer, uncovered, for 30 or 40 minutes or until sauce is thickened. Set aside to cool. Process about 2 cups at a time. Serve at once or cover and chill up to 3 days. Reheat before serving, or let stand at room temperature for 1 to 2 hours before serving. Fifty servings equals about 15 cups; serve the sauce in small bowls and refill as needed.