Cook shells and spinach according to package directions. Drain and squeeze out excess moisture from the spinach. Meanwhile, brown ground turkey and onions in 2 tbsp. olive oil, over medium-high heat.
Preheat oven to 375°F. Grease or line a large baking dish with aluminum foil. Spread 1 cup sauce on bottom of baking dish.
Combine spinach, parsley, crumbs, ricotta cheese, eggs, garlic, and cheese in a large bowl; mix well. Add turkey and onions, and mix well. Fill shells with this mixture.
Add shells and cover with remaining sauce. Top with mozzarella cheese. Cover with foil. Bake 35-40 minutes until warmed through and bubbly.