Preheat oven to 375°F.
In a large skillet over medium heat, cook ground beef until browned, about 7-8 minutes, breaking it up as it cooks. Drain excess fat well.
Add chopped onion and minced garlic to the skillet. Cook 3-4 minutes until onion is softened.
Stir in salt, pepper, and thyme. Remove from heat and let cool 5 minutes.
In a large bowl, combine cooled meat mixture with all three cheeses. Mix until well combined.
Unroll one pie crust and press into a 9-inch pie pan.
Add meat and cheese filling to the crust, pressing down gently to pack it in evenly.
Place second pie crust on top. Crimp edges together with your fingers to seal.
Brush top crust with beaten egg. Cut 3-4 small slits in the top to allow steam to escape.
Bake 30-35 minutes until crust is golden brown and filling is bubbling.
Let cool 10 minutes before slicing and serving.