Cheeseburger Mac

Ingredients

• 1 pound ground beef
• 2 1/2 cups milk
• 1/4 cup ketchup
• 2 tablespoons KRAFT Pur Prepared Mustard
• 1 tablespoon onion powder
• 1 package (12 oz.) VELVEETA Shells & Cheese Dinner
• 1/4 cup Breakstone's or Knudsen sour cream
• 1/4 cup sliced green onions
• 1 cup chopped tomato (about 1 large)