Unsalted butter, for greasing
8 ounces macaroni
2 1/2 cups cold milk
4 ounces provolone cheese, coarsely grated
4 ounces aged Asiago cheese, coarsely grated
2 large eggs
2 scallions, white and green parts, chopped
3 slices white sandwich bread, torn into small pieces
3 strips bacon
1 medium onion, chopped
1 clove garlic, minced
3 tablespoons all-purpose flour
1/8 teaspoon cayenne pepper
Kosher salt