Turkey-Pastrami Croque-Madame Casserole

Preheat oven to 450 degrees. Trim ends of bread so it fits in a 9-by-13-inch baking dish, then toast directly on oven rack until crisp, 8 to 10 minutes. Melt 3 tablespoons butter in a saucepan over medium-high heat. Add flour; cook, whisking, until golden, 2 minutes. Whisk in milk. Boil, still whisking, until thickened, 2 minutes. Remove from heat. Whisk in Dijon, 1 cup cheese, 1 teaspoon salt, and 1/4 teaspoon pepper. Spread 3/4 cup cheese sauce evenly in bottom of dish. Nestle bread into sauce, cut-side up. Top with pastrami and remaining cheese sauce. Sprinkle with remaining 3/4 cup cheese.

Bake until bubbly and browned in places, 15 to 18 minutes. Fry eggs in remaining 1 tablespoon butter in a large nonstick skillet over medium-high heat until whites are set but yolks are still runny, 2 to 3 minutes. Top toasts with eggs; sprinkle with chives and serve.