Truffle Potato Gratin

Ingredients

• 1 tablespoon butter, for preparing the dish
• 3 pounds Idaho potatoes
• 3 teaspoons salt, divided
• 1 tablespoon olive oil
• 1 tablespoon truffle oil, plus more for drizzling
• 2 yellow onions, sliced
• ½ cup chicken broth
• 1 ½ cups heavy cream
• 1 bunch thyme, reserving a few sprigs for garnishing
• 2 bay leaves
• 1 ½ cups shredded Gruyere cheese
• ¼ cup creme fraiche
• Salt
• pepper
• Chopped chives and thyme leaves for garnishing
• Flaky sea salt for finishing