Bring a large pot of water to a boil over medium-high heat. Once the water is boiling, salt liberally.
In a pot of salted, boiling water, boil the shells until al dente, or until they are cooked through with a slight bite to them. Drain the cooked pasta, and set aside. You do not need to keep any pasta water before draining.
1 lb Medium shells pasta
In a large pot, melt the butter over medium-low heat. Once the butter is melted, sprinkle in the all purpose flour. Combine together, and let it cook through for 1-2 minutes, or until the flour is a medium-blonde colour.
2 tablespoon Butter, 2 tablespoon All purpose flour
Add the whole milk and half and half cream to the pot in a slow stream while whisking vigorously, and let it come to a simmer. Continue to stir consistently. As it heats, the sauce will thicken. Once the milk has come to a thicker sauce consistency, add your spices.
1 cup Whole milk, 1 cup Half and half cream, ½ teaspoon Garlic powder, ½ teaspoon Onion powder, ½ teaspoon Mustard powder
Reduce the heat to low. In small portions, add the cheese to the sauce. Add a small handful amount, stir to combine, and repeat until all the cheese is mixed in. Once the cheese has melted, add the pasta, and combine well.
1 ½ cups Sharp cheddar cheese, ½ cup White cheddar cheese