Preheat the oven to 350 degrees Fahrenheit and grease a 2-quart or 8 x 8 cake pan.
Cut off the top of the apple and then use a melon baller or spoon to scoop out the core and seeds of the apple, being sure to leave about 1/2 inch of perimeter of the apple intact. Use a pastry brush to brush the inside of the apple with the lemon juice.
Chop up the top of the apple and any remaining flesh of the apple.
In a small bowl, mix together brown sugar, oats, walnuts, cinnamon, nutmeg, salt, and butter until combined and crumble like consistency is reached. Stir in the chunks of chopped apples.
Pack the cavity of the apple with the crumble mixture.
Place the apples in the prepared dish. For SOFT apples, add 1/2 water of water to the pan, being careful to pour the water around the apples, as to not get water into the filling of the apples.
Bake for 45-50 minutes, until the apples, are cooked through and tender, yet not mushy, and the top of the filling is golden brown.
Serve apples warm plain, with yogurt, or with ice cream.
Apples: Use firm apples, like Gala, Honeycrisp, Braeburn, Granny Smith, or Pink Lady.Gluten-Free Baked Apples: Check to ensure your oats are certified gluten-free.Dairy-Free Baked Apples: Use your favorite non-dairy butter, such as Earth Balance. Serve with non-dairy ice cream.Nut-Free Baked Apples: Instead of chopped walnuts use an additional 2 tablespoon of oats.The majority of sugar comes from the apple itself. This dessert only has 8.5 added grams of sugar.