In a skillet over medium heat, brown the ground beef (or turkey) with the diced onion and minced garlic until the meat is no longer pink and the onion is translucent. Drain excess fat if necessary.
Transfer the cooked meat mixture to the crockpot.
Stir in the taco seasoning, black beans, corn, diced tomatoes, tomato sauce, and beef broth. Mix well to combine.
Cook on high for 4 hours or low for 6 hours.
About 30 minutes before serving, break the spaghetti in half and add it to the crockpot along with 1 cup of water (or additional beef broth). Stir gently and cook until the spaghetti is tender, about 25-30 minutes.
Once the spaghetti is cooked, sprinkle the shredded cheddar cheese on top, cover, and let it melt for about 5 minutes.
Stir before serving, and season with salt and pepper to taste.