In a large skillet over medium heat, cook the ground beef or turkey with chopped onion and green bell pepper for 8–10 minutes, breaking up the meat as it cooks. Drain excess fat once the meat is no longer pink.
Stir in diced tomatoes with green chilis, tomato sauce, mushrooms, minced garlic, oregano, sugar, and salt. Mix well until everything is combined.
Reduce heat to low, cover the skillet, and simmer for 45 minutes. Stir every 10–15 minutes to prevent sticking. The sauce should thicken and develop rich flavor.
While the sauce simmers, boil a large pot of salted water. Add spaghetti and cook until al dente according to package directions. Drain and set aside.
Spoon the meat sauce over the spaghetti. Top with Parmesan cheese if you like. Serve hot and enjoy!