Preheat oven to 400°F. Make filling: In a skillet over medium-high heat, sauté beef, onion, garlic and mushrooms until beef is no longer pink, 7 minutes. Transfer mixture to a bowl. Stir in spinach, Worcestershire, mustard and cheese. Season with salt and pepper. Set aside.
Make topping: Melt butter in a pan over medium heat. Sprinkle in flour and cook, stirring, until pale golden, about 2 minutes. Whisk in milk; bring to a boil, whisking. Cook and whisk until thick and smooth, about 2 minutes. Whisk in Parmesan; season with salt.
Lightly beat yolks in a bowl. Slowly whisk in half of sauce. Pour into pan, whisking. Whisk over medium-low heat until thick, about 2 minutes. Transfer to a bowl.
Line a baking sheet with foil; place shell in center. Stir 3 eggs into meat mixture; spread in shell. Spread topping over mixture. Bake until top is puffed and brown, about 35 minutes. Let rest 10 minutes.
This recipe was previously published in All You, May 2013.