Preheat the oven to 350°F. Lightly grease an 8×8-inch baking dish.
In a large bowl, toss the apple with the lemon juice. Add the sugar, flour, cinnamon, and nutmeg and toss until well combined. Spread into an even layer in the baking dish. Sprinkle it with the butter.
In a medium bowl, stir together the oats, flour, sugar, cinnamon, and salt. Add the cold cubed butter to the mixture and work together with your hands or a fork until the mixture is crumbly and has clumps about the size of peas. Sprinkle the topping all over the top of the apples.
Bake for about 45 minutes or until the top is golden brown and the juices are bubbling. Cool for a few minutes and serve warm with vanilla ice cream if desired.