Instructions

Preheat the oven to 400 degrees F (200 degrees C).
Add ground beef to a skillet and season with salt and pepper. Cook and stir beef over medium-high heat until browned and crumbled, about 10 minutes. Remove to a strainer and set aside. Pour oil into the same skillet and heat over medium heat. Add onion and cook until soft and translucent, 4 to 5 minutes. Add garlic and cook until fragrant, about 30 seconds.
Return beef to the skillet. Pour in marinara sauce, mix to combine, and turn off the heat.
Mix ricotta cheese, Parmesan cheese, 1/2 cup mozzarella cheese, and egg together in a bowl. Place the meat mixture in an 8x8-inch oven-safe casserole dish. Spread the cheese mixture over the meat and sprinkle with remaining 1 cup mozzarella cheese.
Bake until casserole is golden brown and cheese is melted, 20 to 25 minutes. Let rest 10 minutes before serving