Preheat oven to 350F.
Grease muffin tin or line with muffin cups.
Whisk together leftover oatmeal, eggs, oil, milk and brown sugar until smooth and there are no clumps of oatmeal.
In a separate bowl, whisk together the flour, salt, baking powder, and cinnamon.
Fold the flour mixture into the wet mixture until combined – being careful not to over mix. (If adding in stir-ins, add them here.)
Spoon into prepared muffin cups.
Place on the middle rack of the oven and bake for 25-30 minutes (closer to 30 minutes if you are using the large muffin pan).
Cool the muffins in the pan before removing.
