1. Combine diced tomatoes, tomato paste, onion, Italian seasoning, dried basil, salt, pepper, garlic, brown sugar, red pepper flakes, and balsamic vinegar a slow cooker. Blend with an immersion blender until smooth.
2. Defrost spinach in microwave (mine takes 4-5 minutes on a very low setting, but times will vary) and squeeze dry of excess water.
3. Combine turkey, spinach, bread crumbs, garlic, salt, pepper, egg, smoked paprika, and parsley in a large bowl, and mix with your hands until combined. Use a 1/8 measuring cup to measure out portions of the turkey mixture and roll into balls.
4. Carefully place each of the meatballs in the sauce. Put the lid on the slow cooker and cook on low for 8 hours.
5. If possible, gently stir mixture every few hours. If not, stir the mixture after its finished cooking and serve immediately.