Preheat the oven to 200C / 180 Fan / Gas 6.
Peel and chop the apples and place them in a large pan with the water and sprinkle over the 3 tbsp of sweetener.
Heat the apples over a medium heat for 5-10mins, stirring occasionally until they soften and start to break down.
Transfer the apple to a large ovenproof dish, spread them out evenly with the back of a spoon and put to one side.
Then to make the topping, place the flour, sweetener, cinnamon & cubed chilled butter into a large mixing bowl and using your fingers rub it together until it resembles breadcrumbs.
Then use you hands to sprinkle the crumbs over the top of the apple, pressing it down very lightly to make it even, but not so hard that it compacts.
Then bake in the pre-heated oven for about 40mins until it's golden on top and the crumble topping is coming away from the edges of the dish.