Cook the pasta according to the package directions to al dente, then drain.
1 lb thin spaghetti
While pasta cooks add the butter to a large pot set over medium heat. Once melted add the flour and whisk together to evenly combine. Cook for 60 seconds to cook off the flour taste.
1/4 cup butter, 1/4 cup flour
Slowly pour in the milk, then the cream, whisking them in as you do. Then add the mustard, garlic and onion powder, and pepper. Whisk together until evenly combined. Cook for 5 minutes, whisking constantly, just until thickened.
2 12 oz cans evaporated milk, 1 cup heavy whipping cream, 2 tsp ground mustard, 1 tsp garlic powder, 1/2 tsp onion powder, 1/4 tsp cayenne pepper
Add all of the cheeses to a large mixing bowl, and mix together until combined.
2 cups shredded fontina cheese, 2 cups shredded sharp cheddar cheese, 2 cups shredded Swiss & gruyere cheese blend
Add 3/4's of the cheese mixture to the cream and cook, stirring often, until melted and smooth. Add the pasta to the cheese sauce and gently stir until completely incorporated together.
Transfer the spaghetti mac and cheese to a 9x13" baking dish and spread out in an even layer. Sprinkle the remaining cheese evenly out overtop.
Bake at 400° for 15 - 20 minutes, just until hot & bubbly. Carefully remove the hot dish from the oven and let it rest for 5 minutes.
Serve warm & enjoy!