Preheat oven to 350 degrees F.
In a medium-sized saucepan, add butter and garlic. Sauté garlic for a couple of minutes or until fragrant. Add flour and whisk swiftly until a smooth paste forms
Gradually add milk, continuously whisking. Cook the mixture over medium heat, whisking frequently, until it starts to thicken.
Once it has thickened, add in grated cheeses. Stir together until cheese is completely melted. Season with ground mustard, salt and pepper.
Add tortellini to the sauce, stirring to coat. Then, pour into 6 greased oven-safe baking dishes (or one big oven-safe baking dish). Bake in preheated oven for 10-15 minutes until bubbly. Broil on high for 3-4 minutes to brown. Serve immediately.