Potato Leek Gratin

Bahan-bahan (4 porsi)

• 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 g reen onion/scallion ((chopped, to garnish))