Meanwhile, in 12-inch skillet, melt butter. Add onions; cook and stir 4 minutes. Stir in garlic; cook 1 minute. Sprinkle flour over onion and garlic; stir with whisk to combine. Cook about 1 minute. Slowly beat in milk. Cook until thickened and bubbly. Stir in 2 1/2 cups of the cheese; cook and stir until melted. Stir in vegetable puree.