Heat oil in skillet over medium heat. Add celery, onion, and garlic; cook until softened, 5 to 7 minutes. Remove from heat.
STEP 3
Mix ketchup, balsamic vinegar, and soy sauce in a small bowl.
STEP 4
In a large bowl, combine turkey, quinoa, eggs, parsley, sage, black pepper, and salt. Add celery mixture and 1/3 cup of the ketchup mixture; mix well.
STEP 5
Spread 1/3 cup of ketchup mixture in two baking dishes. Form turkey mixture into meatballs with damp hands. Pour remaining ketchup mixture over meatballs.
STEP 6
Bake until meatballs are cooked through and sauce thickened, 15 to 20 minutes.