4 tablespoons salted butter, softened, divided 1 ½ tablespoons all-purpose flour 1 cup milk 2 ounces shredded Gruyère cheese ¼ teaspoon salt, or to taste ¼ teaspoon ground black pepper, or to taste ⅛ teaspoon ground nutmeg 4 slices white bread, lightly toasted 2 teaspoons Dijon mustard 2 thin slices Black Forest ham 2 large eggs 1 tablespoon water 1 teaspoon chopped fresh dill, or to taste (Optional) Set an oven rack about 6 inches from the heat source and preheat…