Combine butter, onion, garlic, and peppers in a 4 cup microwavable casserole. Microwave uncovered at High for 2 3 minutes or until softened. Blend in flour and microwave at High for 30 seconds. Gradually whisk in milk until smooth. Add tomatoes; microwave, covered at High for 3 5 minutes or until mixture comes to a boil and thickens, whisking once. Stir in cheese until melted. Serve warm with tortilla chips or veggies. Makes 2 cups.