Preheat oven to 375 degrees F. Place apple slices in a 2-quart square baking dish; set aside. In a small bowl stir together the 2 tablespoons flour and the 1 tablespoon brown sugar until well combined. Sprinkle flour mixture over apple slices in dish; toss to coat.
Bake, covered, for 15 minutes. Meanwhile, in a medium bowl combine rolled oats, the 2 tablespoons flour, and the 2 tablespoons brown sugar. Using a fork, stir in peanut butter until mixture resembles coarse crumbs. Stir in peanuts. Uncover apple mixture; sprinkle with oat mixture.
Bake, uncovered, for 15 to 20 minutes more or until apples are tender and topping is golden. Serve warm.