Heat a drizzle of olive oil in a skillet over medium heat. Add the onion, garlic, celery, and beef. Sauté for 5–8 minutes until it’s browned.
Add the cooked meat into the Crock Pot along with the oregano, thyme, ziti, crushed tomato, beef stock, and a pinch of salt and pepper.
In a bowl, mix the egg with the shredded mozzarella, ricotta, and a pinch of salt and pepper.
Spread the ricotta mix over the meat and ziti mix. Set the Crock Pot on HIGH for 1.5–2 hours or until the pasta is cooked and the cheese oozes.