Cook onion and Italian dressing in a small sauté pan over medium heat for 5 minutes. Remove from heat and let cool slightly, about 5 minutes.
Combine turkey, bread crumbs, garlic, sautéed onions, Italian seasoning, salt, and pepper in a bowl; mix well.
Combine diced tomatoes, marinara sauce, and dried basil 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 diced tomato mixture over meatballs. Form remaining turkey mixture into meatballs and layer with remaining tomato mixture.
Cover and cook on Low for 8 hours or High for 4 hours.