Make the meatballs by combining the grated onion, nutritional yeast, salt, garlic powder, flour, and turkey in a medium bowl. Stir until well combined, being careful not to over-mix. Form into meatballs, about 1 teaspoon each.
Add the onion, carrots, celery, zucchini, diced tomatoes, chicken stock, Italian seasoning, and salt to a 6-quart slow cooker, stirring to combine.
Add the meatballs, pressing gently to submerge in the liquid. Cover and cook on high for 4-6 hours or low for 6-8, or until the vegetables and meatballs are cooked through.
Uncover, add the spinach, and cook on high for 5 more minutes, or until the spinach is wilted. Add the lemon juice, stirring to combine. Taste and add additional salt if desired.
Serve with a sprinkle of fresh rosemary and good drizzle of olive oil.