Melt the butter down completely over medium heat. Add the flour and whisk together until the flour is completely mixed in and you have a thick gravy-like consistency, 5 minutes.
Combine the milk and cream in a saucepan over medium heat, and when the milk is boiling, slowly add in the butter-flour mixture and cook, stirring, until it has thickened into a sauce. Add salt and pepper to taste. Reserve 10 ounces of the bechamel sauce for the mac and cheese; set aside the remaining bechamel for another use.
For the mac and cheese:
Preheat the broiler.
Fry the bacon in a small pan until crisp. Remove the pieces with a slotted spoon.
Add the reserved bechamel sauce to a 12-inch skillet over medium heat; mix in the macaroni, Cheddar, fontina and bacon. Add salt and pepper to taste. Transfer the mixture to a 16-ounce casserole dish (single-serving size).
For the topping:
Top the casserole with the Cheddar, fontina, fried potato and egg. Cook under the broiler until the topping looks golden brown.
Garnish with scallions and freshly ground black pepper and serve.