Preheat the oven to 350°F (180°C) and place the rack in the center so the crumble bakes evenly.
Peel the apples, cut them into small half-inch cubes, and place them in a large mixing bowl.
Add the flour, sugar, and cinnamon to the apples, pour in the lemon juice, then toss everything together until the pieces are evenly coated, and spread the mixture into a 1.5-quart baking dish.
In another bowl, mix the oats, flour, brown sugar, baking powder, cinnamon, melted butter, and salt until the mixture looks like damp sand with little clumps forming.
Sprinkle the crumble mixture evenly over the apples, breaking apart any big clumps with your fingers so the topping covers everything well.
Bake the crumble for 30 to 40 minutes, until the top is golden brown and the filling is bubbling around the edges.
Remove from the oven, cover loosely with foil to keep warm, let it rest for 10 minutes so the apple syrup thickens slightly, then serve warm with vanilla ice cream.