Steps
1. Preheat the oven to 375°F (190°C).
2. Cook the pasta according to package instructions and drain.
3. In a bowl, mix the cooked pasta, tuna, cheese, and ketchup.
4. Transfer the mixture to a baking dish and top with sliced onions.
5. Bake for 20-25 minutes until the cheese is melted and bubbly.
6. Serve hot and enjoy!