Place a skillet over medium heat and add 1 tablespoon of butter. When it foams add the shrimp.
Cook the shrimp for 2-4 minutes or until they just turn pink; Remove the shrimp from heat and reserve.
Cook the macaroni per the package directions. Drain and reserve.
Heat the remaining 2 tablespoons of butter in a nonstick saucepan. When the butter melts, add the flour and whisk to combine. Cook the butter/flour mixture for 2-3 minutes over medium heat.
Slowly whisk in the milk and cook over medium heat until it comes to a boil. Remove pot from heat.
Add a pinch of nutmeg, a few grinds of black pepper and a good pinch of salt to the milk mixture.
Add the grated cheeses and cover the pot. Allow it to sit for 3 minutes or until the cheese has melted.
Remove the lid and add the cooked macaroni and reserved shrimp. Stir to combine.
Serve:
Serve in warm bowls, top with some green onions and additional BBQ spice.