Heat a large skillet over medium heat. Add the ground turkey and cook, breaking it up with a spatula or fork, until it loses its pink color.
Drain any excess fat from the cooked meat, then transfer the meat to the slow cooker.
Add the broken whole-wheat spaghetti and the basil marinara sauce to the slow cooker. Stir well to make sure the pasta is coated in sauce.
Cover and cook on low for 2–3 hours (about 120–180 minutes), or until the spaghetti is al dente and cooked through. Stir once halfway through if possible to prevent sticking.
Transfer to a serving platter or bowls and sprinkle with the freshly grated Parmesan before serving.
Notes
Use ground beef instead of turkey if preferred.
Break the spaghetti into pieces so it fits and cooks evenly in the slow cooker.
Stir once during cooking to prevent pasta from clumping.
Cook until pasta is al dente to avoid a mushy texture.