In a large saucepan melt the butter over low heat; add garlic and saute until fragrant. Whisk in flour and cook 3-4 minutes, whisking constantly. Slowly whisk in milk, mustard and season with salt and pepper, bring to a simmer 4 minutes, whisking occasionally. Add 2 cups of cheddar cheese and parmesan; stir until mostly melted; remove from heat.