Preheat oven to 200 degrees celsius (180 if using fan-forced).
Grease a large baking dish with butter and set aside.
Place the peeled and sliced apples into a medium saucepan with the water, caster sugar, lemon juice, cinnamon and nutmeg.
Bring to a low simmer for 5-10 minutes or until just softened. Drain off the excess liquid. Place the stewed apples into the baking dish.
To make the crumble, place the oats, brown sugar, plain flour and butter into a bowl. Use your fingers to squeeze and break up the lumps of butter until crumbs have formed and all of the oats and flour are completely combined with the butter (there should be no loose bits left!).
Sprinkle the crumble over the apple and place into the preheated oven.
Cook for 35-40 minutes or until the crumble is golden and crunchy.
Serve immediately with ice-cream or cream.