Preheat the oven to 350 degrees F (180 degrees C).
Then get the crisp topping mixture ready. Combine All-Purpose Flour (1/4 cup), Oats (1/2 cup), Brown Sugar (1/2 cup), Ground Cinnamon (1/2 teaspoon), and Ground Nutmeg (1 pinch) in a mixing bowl.
Add the Butter (3/4 cup) and mix with fingers until the mixture is crumbly. Don't overmix; it's okay to have small chunks of cold butter in this mixture.
Put this topping mixture in the fridge to keep the butter cold.
Next, cut the tops off Red Apple (6), then remove as much of the core as you can. Don't remove too much of the red apple pulp. Keep at least a quarter inch of pulp from the red apple skin.
Peel the skin off the Green Apple (2) then cut them into small dices (discard the center core). Pour Ground Cinnamon (1/4 teaspoon), Ground Nutmeg (1 pinch), and Granulated Sugar (1 teaspoon) over the diced green apple. Mix well.
Line a baking sheet with parchment paper. Arrange the red apples on the baking sheet. Fill the red apples with the diced green apple, then top them with the crispy topping mixture.
Bake for 45 minutes or until the crispy topping is golden brown. Serve and enjoy them while they're still warm.