Preheat the oven to 400°F. Grease a 12 cup muffin tin.
Whisk together the oats, brown sugar, cinnamon, nutmeg (if using) and salt in a large bowl.
Whisk together all remaining ingredients (EXCEPT chocolate chips and walnuts) in a large measuring jug.
Pour wet into dry ingredients, stir well and fold in chocolate chips and walnuts.
Divide mixture between muffin cups, gently shake them to level out. Either cover and refrigerate overnight or bake right away for 20 minutes or until the top is golden brown.
Oats: I recommend regular rolled oats. If you want to use quick oats, you'll need to add a couple of tablespoons extra and the oatmeal will be a little different in texture. Steel cut oats do not work.
Honey: A mild, runny honey works best here. You can also use maple syrup instead.
Brown sugar: If you want to make these refined sugar free, leave out the brown sugar and add an extra tablespoon of honey instead.
Milk: Any milk works - whole, skim, almond... Pick your favorite.
Oil: I like using melted coconut oil here, but you can also use a neutral vegetable oil or melted butter.
If refrigerating overnight, baking time increases by about 5 minutes.To freeze, pack the completely cooled oatmeal cups into single serve freezer bags or into freezer containers without touching. To defrost, simply take out however many you need and thaw in the fridge overnight. Reheat in the microwave or oven.