Instructions: 

1. Preheat oven to 350 degrees.
2. Bring a pot of water to rapid boil, add pasta and reduce to a gentle simmer. Cook until just on the firm side of al dente, about 6-8 minutes. (Shells will soften during baking).
3. Melt butter in a large saucepan. When melted, add flour and blend to form a roux. When roux smells slightly nutty, add 2 cups milk and whisk to combine. When sauce begins to thicken, season with salt and mustard. Taste and adjust.
4. Add all cheeses (except reserved cheddar cheese) to sauce, whisk until combined.
5. Add shells to sauce and coat thoroughly.
6. In a medium skillet, heat a tablespoon of oil. Reduce heat to low and add garlic. Stir until aromatic.
7. Add breadcrumbs to skillet, toss until toasted.
8. Pour shells and sauce into a casserole dish. Top with reserved grated cheddar and breadcrumbs.
9. Bake at 350 degrees for 20-30 minutes, or until cheddar bubbles and shells have softened.