Preheat oven to 375. Peel, core and cut apples into wedges, halve wedges crosswise so pieces are smaller than 2 inches. Put apples in large bowl. Squeeze juice from lemon, retaining as much pulp as possible. Add lemon juice and pulp to apples; toss gently to coat. Sprinkle with sugar and flour; toss gently to mix. Transfer mixture to an ungreased 3 quart rectangular baking dish.
Arrange doughnuts on top of apple mixture; (I would use cinnamon doughnuts if apple doughnuts aren't available). Drizzle with melted butter. Bake, uncovered, about 45 minutes or til filling is bubbly. If needed, to prevent over browning, cover loosely with foil for the last 10 to 15 minutes of baking. Serve warm.