Preheat oven to 375°F. Lightly grease a 9-inch square baking dish with butter or cooking spray.
Toss the sliced apples with lemon juice in a large bowl so they don't brown while you work. Add sugar, cinnamon, nutmeg, and cornstarch; stir until the apples are evenly coated. Spread into your prepared dish.
In another bowl, whisk together flour, rolled oats, brown sugar, and salt. Cut the cold butter into small cubes and add to the mixture. Use your fingers or a pastry cutter to work the butter into the dry ingredients until you have a coarse texture with some pea-sized bits remaining—this is what gives the topping its crumbly character.
Drizzle heavy cream over the flour mixture and stir gently until it comes together in clumps. Don't overmix; you want irregular pieces, not a uniform dough.
Scatter the topping evenly over the apples. For extra crunch, press down lightly with your hands to create some larger clusters, then let them break apart as they bake.
Bake for 40-45 minutes until the top is golden brown and you can see bubbles forming at the edges where the apple juices are escaping. If it browns too quickly after 30 minutes, tent loosely with foil.
Let rest for 15 minutes before serving—the filling will be very hot initially and needs time to set so it doesn't run everywhere when you scoop.
