In a small pan over medium heat in reserved hot bacon drippings, cook onion until tender, about 5 minutes. Add garlic and cook 1 minute more. Cut into 3-inch circles using a biscuit cutter or glass dipped in flour. Place 1 teaspoon of the spinach filling on one half of a circle. Using your fingertip, moisten edge of circle with a little water.
Place empanadas on a cookie sheet and freeze.