Prep the apple. Core and dice one medium apple into small cubes so it cooks quickly.
If you like a softer texture, peel it first; if you like more bite and fiber, keep the peel on.
Warm the base. In a medium saucepan, add 1 cup milk and 1 cup water. Stir in a pinch of salt and 1 teaspoon ground cinnamon. Warm over medium heat until steaming but not boiling.
Cook the apples. Add the diced apple to the pot and simmer for 2–3 minutes, stirring occasionally, until slightly softened and fragrant.
Add the oats. Stir in 1 cup rolled oats.
Reduce heat to medium-low and cook for 5–7 minutes, stirring often, until creamy and thickened. If it looks too thick, splash in more milk or water.
Sweeten and flavor. Stir in 1–2 teaspoons maple syrup or honey and 1/2 teaspoon vanilla extract, if using. Adjust cinnamon to taste.
Add the protein. Remove the pot from heat.
If using protein powder, let the oatmeal cool for 1 minute, then whisk in 1–2 scoops until smooth. If using Greek yogurt, fold in 1/2–3/4 cup until creamy.
Finish with toppings. Add chopped nuts for crunch and healthy fats. Sprinkle chia or flax for extra fiber and omega-3s.
Top with a few fresh apple slices if you like.
Serve warm. Taste and adjust sweetness or cinnamon. Enjoy right away, or portion into containers for later.