Adjust oven racks to top and bottom positions and heat oven to 500 degrees.

Arrange potatoes on rimmed baking sheet, pour ¾ cup water into baking sheet, and wrap tightly with aluminum foil.

Cook on bottom rack until paring knife or skewer slips in and out of potatoes easily (poke through foil to test), 25 to 30 minutes.

Remove foil and cool 10 minutes. If any water remains on baking sheet, blot dry with paper towel.

Drizzle 3 tablespoons oil over potatoes and roll to coat.

Space potatoes evenly on baking sheet and place second baking sheet on top; press down firmly on baking sheet, flattening potatoes until 1/3 to 1/2 inch thick.

Sprinkle with thyme leaves and season generously with salt and pepper; drizzle evenly with remaining 3 tablespoons oil.

Roast potatoes on top rack 15 minutes. Transfer potatoes to bottom rack and continue to roast until well browned, 20 to 30 minutes longer.

Serve immediately.