1. Peel potatoes and drop in cold water (or potatoes may be left unpeeled); slice each potato 1/4-inch thick, dropping each slice into a bowl of fresh cold water.
2. Line a 15-by-10-by-1-inch jelly roll pan with foil.
3. Place butter, garlic, thyme and onion on foil and heat in 450-degree oven until butter is melted; stir and leave oven on.
4. Drain potatoes and arrange on butter mixture, turning to coat both sides. Overlap slices as necessary.
5. Bake for 15 minutes; take from oven and drizzle with lemon juice and sprinkle with Parmesan.
6. Bake until edges are crisp — 10 to 15 minutes.
7. Serve at once.