• Gather all the ingredients.
  
• Rinse the leeks and cut in half lengthwise. Chop into 1” (2.5 cm) pieces.
  
• Carefully wash the leeks again in running water.
  
  
• Butter a baking dish.
  
  
• In a large skillet, heat the rest of the butter on medium-high heat and add leek.
  
• Cover and let simmer for 20 minutes. Stir occasionally and cook until leeks are soft.
  
• Meanwhile, peel and slice the potatoes into ¼” (1.3 cm) pieces.
  
• Soak in water until you are ready to use them (prevent from changing color).
  
• When the leeks are soft, add thyme, salt, pepper, and heavy whipping cream. Simmer another 5 minutes until the sauce gets thickened.
  
• Preheat the oven to 400°F (200ºC). Divide the potatoes, leek mixture, and each cheese according to your baking dish size. I divided them in half as my baking dish is shallow.
• 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 panko and green onion on top. Cover with the aluminum foil and bake for 45 minutes. To check whether the dish is done, test by inserting a skewer through the foil. If the skewer is able to go through the potato smoothly then it’s ready. Remove the cover 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.
  

To Store

• You can keep the leftovers in an airtight container and store in the refrigerator for up to 3 days. The potato texture will change when frozen, so I do not recommend freezing this dish.