Preheat oven to 375 degrees F. Peel, core, and cut apples into wedges; halve wedges crosswise so the pieces are smaller than 2 inches. Juice the lemon, retaining as much pulp as possible; toss juice and pulp with apples. Sprinkle with sugar and flour; toss until apples are coated. Arrange apples in a 3-quart rectangular baking dish.
Top apples with doughnuts and drizzle with melted butter. Bake about 45 minutes or until filling is bubbly, covering with foil if necessary to prevent overbrowning. Serve warm.