• Microwave 1½ lb kabocha squash for 2 minutes and remove the seeds and pith; learn how to cut it open in my post about kabocha squash.
  
• Cut into wedges and remove the skin.
  
• Cut into small chunks, about 1-inch (2.5-cm) cubes.
  
• Thinly slice ½ onion and 1 clove garlic.
  
• Discard the bottom of 3.5 oz shimeji (brown beech) mushrooms. Separate the shimeji mushrooms into small pieces with your hands.
  
• Cut 9.5 oz king oyster mushrooms (eringi) into 2-inch (5-cm) pieces and slice thinly.
  

To Make the White Sauce

• In a large saucepan, melt 4 Tbsp unsalted butter over low heat. Once the butter is completely melted, add ½ cup all-purpose flour (plain flour) and mix well until blended. Cook the flour, stirring constantly, without browning, for 1–2 minutes.
  
• Gradually add in 3⅓ cups whole milk, ½ cup at a time, while stirring constantly until the sauce is completely smooth and without any lumps.
  
• Season with a pinch of Diamond Crystal kosher salt, ⅛ tsp white pepper powder (or freshly ground black pepper), and ¼ tsp freshly grated nutmeg.
  
• Cook the white sauce, stirring constantly until the sauce has thickened, about 10–15 minutes. It should be a thick, creamy sauce. Taste the sauce one more time and adjust the seasonings to make sure it’s not bland. Set aside.
  

To Cook the Kabocha

• In a large frying pan, heat 1½ Tbsp extra virgin olive oil. Add the garlic and onion and sauté until the onion is coated with oil.
  
• Add the oyster and shimeji mushrooms and cook for 2–3 minutes.
  
• Once the mushrooms are tender, add 1½ Tbsp miso and mix well together.
  
• Add the kabocha and 1 cup water for steaming. Close the lid and cook kabocha on medium-low heat until it’s tender, about 10 minutes.
  

To Cook the Macaroni (optional)

• Bring a large pot of boiling water to a rolling boil over high heat. Add 2 Tbsp Diamond Crystal kosher salt and 1½ cups uncooked elbow macaroni. Cook according to the package instructions, but 1 minute less than the recommended time. The pasta will finish cooking in the frying pan, so this keeps it from becoming too soft. Reserve ¼–½ cup pasta cooking water. Drain thoroughly and set aside.
  

To Assemble

• Check the kabocha for doneness by inserting a wooden skewer into a piece. If it slides through easily, it’s ready.

  Add the white sauce and gently stir to coat everything evenly.

  Tip: If the sauce feels too thick, add 1/8–1/4 cup of the reserved pasta water, a little at a time, to loosen it and help the sauce cling to the ingredients.
  
• Taste the sauce and season with Diamond Crystal kosher salt and freshly ground black pepper, if necessary.
  
• Add the cooked macaroni and mix well.
  
• Grease the inside of your 2 8- x 8-inch (20- x 20-cm) baking dishes with a stick of butter (or use 1 8- x 8-inch baking dish and 4 individual gratin dishes). You can also melt 1 Tbsp unsalted butter and use a brush to grease the dishes. Transfer the mixture to the buttered baking dish(es).
  
• Grate ½ cup Gruyere cheese on top and sprinkle ¼ cup panko (Japanese breadcrumbs).
  
• Adjust the oven rack so the food is about 6 inches from the heat source. Place the baking dishes in the preheated broiler or 500ºF (250ºC) oven for 3 to 4 minutes until the top is nicely golden brown and bubbling around the edges. If the dishes are not browning evenly, rotate them around. Remove from the oven and let the gratin stand for 5 minutes. Garnish with chopped chives or parsley and serve.
  

To Store

• You can keep the leftovers in an airtight container and store in the refrigerator for up to 3 days and in the freezer for a month.