Foryoueats: Potato Leek Gratin

Ingredients

• 3 Tbsp unsalted butter
• 2 leeks
• 1 tsp Diamond Crystal kosher salt
• ¼ tsp thyme (fresh or dried)
• ⅛ tsp freshly ground black pepper
• ½ cup heavy (whipping) cream
• 1½ lb russet potatoes (3 potatoes)
• 2 cups Parmigiano-Reggiano or Parmesan cheese (grated)
• 2 cups Mexican-style shredded cheese
• 1 Tbsp panko (Japanese breadcrumbs)
• 1 green onion/scallion (chopped, to garnish)