Preheat your oven to 350°F and lightly grease a 12-cup muffin tin with coconut oil or non-stick spray.
Core and dice the apples into small, uniform pieces, leaving the skin on for extra fiber.
In a large mixing bowl, whisk together the eggs, Greek yogurt, almond milk, and vanilla extract until the mixture is smooth.
Add the rolled oats, vanilla protein powder, ground cinnamon, baking powder, and sea salt to the wet ingredients, stirring until fully incorporated.
Gently fold in the diced apples and crushed walnuts until they are evenly distributed throughout the batter.
Divide the mixture evenly among the 12 muffin cups, filling each nearly to the top and pressing down slightly with a spoon.
Bake for 25 to 30 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
Allow the oatmeal cups to cool in the tin for 10 minutes before transferring them to a wire rack to cool completely.
Store in an airtight container in the refrigerator for up to 5 days, enjoying three cups per serving for a balanced meal.