In a medium saucepan, combine the oats, milk, water, maple syrup, cinnamon, salt, and any optional additions, such as chia seeds, flaxseed, and/or peanut butter. Heat over medium-high until the liquid reaches a low, gentle boil, then reduce the heat to a simmer.
Add the apple. Continue simmering, stirring periodically, until the oatmeal is creamy and has absorbed most of the liquid, about 6 to 8 minutes. Remove from the heat and stir in the vanilla.
While the oatmeal cooks, make the topping: Heat medium skillet over medium low heat. Melt the butter. Add the apples, maple syrup, cinnamon, and salt. Let cook, stirring occasionally, until the apples are soft and tender but not mushy, about 8 minutes. Taste and adjust the sweetness, cinnamon, or salt as you like.
Serve the oatmeal hot, topped with the sauteed apples and pecans.
Notes
*You can sub quick oats, but note that the texture will be noticeably mushier and they will cook more quickly—add the apples in step 1. If you use steel cut oats, you will need to increase the cook time (see How to Cook Steel Cut Oats). Do not use instant oatmeal
TO STORE: Transfer leftovers to an airtight container and refrigerate for up to 4 days.
TO REHEAT: Warm the oatmeal in a microwave-safe bowl with a splash of milk to rehydrate the oats for 30-second increments, stirring after each. You may also warm gently over low heat on the stovetop with a splash of milk until heated through.
TO FREEZE: Freeze in an airtight container or freezer bag for up to 3 months. For ease, if you make the topping, you can just freeze that with the oatmeal (although you can freeze it separately if you prefer.)