Grease a 10-inch (20 cm) pie mold. Cover bottom with thin apple slices.
Sprinkle white sugar and cinnamon on top. Drizzle with lemon juice.
Mix, on a separate bowl, oats, dark sugar and flour with melted butter or margarine.
Cover apples with this mixture.
Bake in preheated oven 300° F (150° C) for 30 to 35 minutes. Remove from oven and cool.
Serve alone or with vanilla ice cream.