Cook pasta shells until al dente, then drain.

Sauté onion and garlic in olive oil, add beef and cook through.

Stir in chopped tomatoes and oregano, simmer for 10 minutes.

Mix ricotta and spinach in a separate bowl.

Spread half the tomato-beef sauce in a baking dish.

Fill shells with ricotta-spinach mixture, arrange on sauce.

Pour remaining sauce over shells, sprinkle with cheddar.