In a large skillet over medium heat, add the ground beef or turkey. Cook with the chopped onion and green bell pepper, breaking up the meat with a spoon as it cooks. Stir occasionally until the meat is browned and the vegetables are softened—about 8–10 minutes. Drain excess fat if needed.

Stir in the diced tomatoes with green chilis, tomato sauce, drained mushrooms, minced garlic, dried oregano, sugar, and salt. Mix thoroughly and bring the mixture to a low simmer.

Cover the skillet with a lid, turn the heat to low, and let the sauce simmer for 45 minutes. Stir occasionally to prevent sticking. The sauce will thicken and the flavors will deepen—look for a deep red hue and a savory aroma.

While the sauce is simmering, bring a large pot of salted water to a boil. Add the spaghetti and cook according to the package directions until al dente. Drain and set aside.

Spoon the meat sauce generously over the cooked spaghetti. Top with grated Parmesan cheese if desired. Serve hot and enjoy immediately.