Beef-and-Spinach Pie

Preheat the oven to 400°F. Prepare the filling: In a skillet over medium-high heat, saute the beef, onion, garlic, and mushrooms until the beef is no longer pink, 7 minutes. Transfer the mixture to a bowl. Add the spinach, Worcestershire, mustard and cheese. Season with salt and pepper. Set aside.Prepare the filling: Melt the butter in a skillet over medium-high heat. Sprinkle with flour and cook, stirring occasionally, until pale golden brown, about 2 minutes. Whisk in the milk; bring to a boil, whisking constantly. Cook, whisking, until thickened and smooth, about 2 minutes. Sprinkle with parmesan cheese and season with salt.Lightly beat the egg yolks in a bowl. Slowly whisk in half of the sauce. Pour into the pan, whisking. Beat over medium-low heat until thickened, about 2 minutes. Transfer to a large bowl.Line a baking sheet with foil; place the eggshells in the center. Mix 3 eggs with the meat mixture; put in the shell. Sprinkle the filling mixture. Bake until the top is puffy and lightly browned, about 35 minutes. Let stand for 10 minutes.