Preheat the oven to 350° F (176°C). Add 1 cup of oats to a blender and blend for 30 seconds, until the oats have been ground into a fine oat flour. (You can also use 1 cup of oat flour instead of whole oats.)
Add the cup of blended oat flour to a mixing bowl along with a cup of whole oats, 1/3 cup brown sugar, 2 tablespoons ground flaxseed, 2 teaspoons cinnamon, 1/2 teaspoon nutmeg, and 1/2 teaspoon salt. Then, mix well.
Peel, core, and dice 2 cups of apples into bite-sized pieces. Then add the apples to the oatmeal mixture and stir well.
Add 2 cups of plant-based milk of choice and mix well. Pour the mixture into a greased baking dish, then set aside while you prepare the crumble topping. (This allows the milk to soak into the oatmeal and soften it a little before baking.)
Make the crumble topping (optional)
In a small bowl, mix the vegan butter, flour, whole oats, brown sugar, cinnamon, baking soda, and salt until well incorporated and crumbly. Sprinkle the crumble mixture on top of the oatmeal. (optional)
Bake at 350°F (176°C) for 35 minutes, until dry and slightly browned on top. Allow the dish to cool for 15 minutes before serving.
Notes
You can adjust the amount of sweetener used to suit your taste. I find that 1/3 cup makes it mildly sweet, which works well when drizzling with extra syrup.
You can also make this dish without apples or other fruit. If you are going to omit the fruit, add an extra 1/4 cup of vegan milk to account for the moisture that the fruit provides.