1. Mix the apple cider vinegar and the milk. Let stand ten minutes to sour.
2. Place the oatmeal in a medium bowl and add the milk. Stir, cover, and set in the refrigerator overnight.
3. In the morning, preheat oven to 375º F.
3. Add baking soda, egg, flour, salt, and sugar to the oatmeal mixture. Mix well, and fill the wells in the mini muffin pan.
4. Bake mini muffins for 13 - 15 minutes. When they are done, the tops should pop back when pressed lightly. Or use the tried and true toothpick method to check.
Notes
This recipe was tested with Bob's Red Mill 1 to 1 Gluten Free Baking Flour. Use gluten free oats if you need them.