Cook pasta shells as directed on the package. Drain and set aside.
Preheat oven to 400°F.
Grate cheese and set aside.
Grease casserole dish and set aside.
In a large saucepan, melt butter over medium heat.
Add flour and whisk together with the butter to make a roux.
Cook stirring constantly until roux turns a light golden brown.
Whisk in red pepper flakes, garlic powder, salt, pepper and dijon mustard.
Slowly whisk in milk and stir until there are no more lumps and sauce is smooth.
Add grated cheese and stir until cheese melts and sauce is smooth.
Stir in heavy cream.
Fold in cooked pasta, stir to coat pasta with the cheese sauce.
Taste and season with additional salt if needed.
Transfer to greased casserole dish and cover pan with foil.
Bake for 30 minutes, until cheese sauce boils.
Remove from oven and let pasta rest for 5-10 minutes before serving.