1. Preheat oven to 350 degrees F (175 degrees C).
2. Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain and set aside.
3. In a large skillet, brown turkey over medium heat. Remove turkey from skillet to a large bowl; do not drain any grease left in skillet.
4. Add onion and bell pepper to skillet; saute until softened. Add garlic to saute for 2 minutes and add tomatoes; mix well and remove from heat.
5. Add vegetable mixture to turkey and stir in ketchup to coat; mix well. In a 9x13 inch baking dish, spread mixture and top with spaghetti and cheese.
6. Bake in preheated oven for about 30 minutes or until heated through and cheese is melted; serve.