Preheat the oven to 350℉.
In a bowl, add the cold and firm coconut oil, flour, brown sugar, oats, walnuts, cinnamon, and a dash of salt. Mix to combine using a fork, pressing and cutting into the coconut oil gently until the mixture is homogeneous and crumbly. Set aside.
Cut apples in half through the core. Use a paring knife or a melon baller to remove the core as well as the stem or stalk. Brush cut and exposed sides of the apple with lemon juice, or use a wedge of lemon and rub it onto the apple.
Place apples onto a baking tray, cut side up. If you find that your apple halves do not balance well, feel free to slice the bottom side so it has a flat surface to sit on, alternatively, lean the apples against one another.
Pack the oat and walnut crumble toppings on top of the apples. Add about ¼-inch of water into the baking tray to help the apple cook and prevent them from sticking. Bake for 35-40 minutes or until the topping is golden brown and the apple is tender.
Optionally serve with a dollop of sweetened whipped coconut cream: refrigerate a can of coconut milk overnight. Carefully open the can and scoop out the separated cream into a bowl, leaving the coconut water in the can. Whip the cream with an electric mixer or a whisk until it starts to hold its shape. Then add 2-3 tbsp. of powdered sugar or monk fruit sweetener and whisk to sweeten the whipped cream. You will only need a tablespoon or two of whipped coconut cream per serving.