Saute onion and Italian dressing in a small saute pan over medium heat for 5 minutes. Remove from heat and let cool slightly, about 5 minutes.

Combine turkey, bread crumbs, garlic, sauteed onions, Italian seasoning, salt, and pepper in a bowl; mix well.

Combine diced tomatoes and marinara in a second bowl; set aside.

Form 1/2 of the turkey mixture into 1 1/2-inch meatballs and layer them in the bottom of a slow cooker. Pour 1/2 of the tomato mixture over the meatballs. Form the remaining turkey mixture into meatballs and layer with remaining tomato mixture.

Cover and cook until no longer pink in the centers, about 4 hours on High, or 8 hours on Low.