Preheat oven to 350˚F/175°C. Lightly grease a 8-inch round or an 8-inch square baking dish. Set aside.
Make The Apple Filling: In a medium bowl, toss apples with lemon juice, sugar, salt, flour, ground cinnamon and nutmeg to coat. Transfer mixture to prepared baking dish and spread evenly. Set aside.
Make The Crumble Topping: In a separate bowl, whisk together the flour, sugars, cinnamon, salt & oats. Then, cut cold butter into small chunks, about 1/2 - 1 inch cubes and add it into the bowl. Mix the butter with the flour mixture by either cutting it in with a pastry cutter or fork, or rubbing it in using your fingers, or using a food processor. In the end, you will no longer have chunks of butter and the mixture will become crumbly. Sprinkle crumble evenly on top of the filling. Press tightly.
Bake for 30-40 minutes, until the topping is golden brown and the filling is bubbling. If the top is browning too fast, cover loosely with aluminum foil. Remove from the oven and allow to cool on a wire rack until warm. Serve warm or serve with vanilla ice cream if you like
Crumble will keep, covered, in the fridge for up to 3 days, but will taste best and be crispier on the first day it’s made.