
Preheat oven to 350°. In a small bowl, combine apple slices, sugar and lemon juice; toss to coat. Set aside. In an 8- or 9-in. cast-iron or other ovenproof skillet, heat butter over medium heat. Add onion; cook and stir until crisp-tender, about 3 minutes. Remove from the heat.
Alternately arrange potato and apple slices in a single layer in same skillet. Combine 3/4 cup Parmesan cheese, cream, thyme, salt and pepper; pour over top.
Bake, uncovered, 50 minutes. Top with bacon and remaining 2 tablespoons Parmesan cheese. Bake until potatoes are tender and top is lightly browned, 5-10 minutes longer. Let stand 10 minutes before serving. If desired, sprinkle with parsley.