Preheat the oven to 350. Peel, core and slice apples. Place the apples in a large bowl; sprinkle them with sugar, lemon juice and cinnamon. Toss the apples to coat evenly. Transfer the apples to a 9-inch square baking dish.
In a small bowl, combine brown sugar, flour and oats. Using a pastry blender or two knives scissor-fashion, cut butter into the flour mixture until it resembles coarse crumbs. Sprinkle the crumb topping over the apple mixture.
Bake the apples until the crumb topping is crisp, about 30 minutes. Spoon the warm apple crisp into small bowls and serve immediately, or let the apple crisp cool and serve it at room temperature.