Gather all the ingredients.

Rinse 2 leeks and cut in half lengthwise. Chop into 1-inch (2.5-cm) pieces.

Carefully wash the leeks again in running water.

Use a bit of the 3 Tbsp unsalted butter to grease a baking dish.

Heat a large skillet on medium-high heat. When it‘s hot, add the rest of the butter. When it‘s melted, add the chopped leeks.

Cover and let simmer for 20 minutes. Stir occasionally and cook until the leeks are soft.

Meanwhile, peel and slice 1½ lb russet potatoes into ¼-inch (1.3-cm) pieces.

Soak the sliced potatoes in water until you are ready to use them (to prevent them from changing color).

When the leeks are soft, add ¼ tsp thyme, 1 tsp Diamond Crystal kosher salt, ⅛ tsp freshly ground black pepper, and ½ cup heavy (whipping) cream. Simmer another 5 minutes until the sauce thickens.

Preheat the oven to 400°F (200ºC). Divide the potatoes, leek mixture, and cheeses (2 cups Parmigiano-Reggiano or Parmesan cheese and 2 cups Mexican-style shredded cheese) according to your baking dish size and depth.

Layer one portion of potatoes in the baking dish.

Then, layer one portion of the leek mixture.

Layer one portion of each cheese. Repeat for each layer.

Sprinkle 1 Tbsp panko (Japanese breadcrumbs) and 1 green onion/scallion (chopped) on top.

Cover with aluminum foil and bake for 45 minutes.

To check whether the dish is done, insert a skewer through the foil. If the skewer is able to go through the potato smoothly, then it’s ready.

Remove the foil and change the oven setting to broil. Broil for 2–3 minutes until golden brown on top.

Remove from the oven and let it sit another 15 minutes. Serve warm.