Roughly chop onion, tomato, and bell pepper. Add to a blender or food processor along with the spinach. Blend until finely chopped, but not pureed. Set aside.
Heat a large skillet over medium. Once hot, add veggies and season with salt and pepper. Cook for 5-7 minutes to soften veggies. Remove from heat and allow to fully cool.
Add turkey, veggies, egg and bread crumbs to a large mixing bowl. Using your hands, fully combine and shape into meatballs. Place into slow cooker and cover with spaghetti sauce. Cook on low for 3-4 hours or until they are no longer pink in the center.
Before serving, prepare fettuccine as directed on package.