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 heat.

Mix ricotta cheese, Parmesan cheese, 1/2 cup mozzarella cheese, and egg together in a bowl.

Place meat mixture in an 8x8-inch oven-safe casserole dish. Spread cheese mixture over meat and sprinkle with remaining 1 cup mozzarella cheese.

Bake in the preheated oven until casserole is golden brown and cheese is melted, 20 to 25 minutes. Let rest 10 minutes before serving