In a large skillet, bring a pot of salted water to a boil. Add spaghetti and cook according to package directions until al dente, about 8-10 minutes. Drain and set aside.

In the same skillet over medium heat, add ground beef or turkey, breaking it apart with a spoon. Cook until browned, about 6-8 minutes. Drain excess fat.

Stir in taco seasoning, and add chopped bell peppers and onion if using. Pour in a splash of water and cook for an additional 2-3 minutes.

Return drained spaghetti to the skillet, tossing into the meat mixture. Combine thoroughly.

Sprinkle cheese over the pasta mixture, cover, and reduce heat to low. Allow cheese to melt for about 3-5 minutes.

Once cheese is melted, fluff the pasta with a fork. Serve hot, garnished with toppings like sour cream or cilantro.