1. Preheat oven to 400 degrees F or 200 degrees C. 

2. Wash the mushrooms and remove any dirt. If the mushrooms you’re using are of a larger variety (like portobello or king trumpet mushrooms), cut in half or into thinner pieces. 

3. In a medium sized bowl mix the miso, mirin, soy sauce, and sesame oil together. Add the sugar if you’d like the dish to have a little more sweetness. 

4. Add the mushrooms to the bowl and mix. Make sure the sauce coats the mushrooms evenly. 

5. Tear off a large piece of aluminum foil and place the mushrooms in the middle. Make a steam bag with the aluminum foil by bringing the edges of the foil together. Fold or crumple the edges together to seal. Make sure there are no open holes or openings that could potentially let steam out. 

6. Place in the oven for 20-25 minutes depending on the size of the mushrooms. When done, remove the aluminum steam bag from the oven. Wait a few moments before opening. Be careful while opening as any steam that escapes will be extremely hot. Garnish with scallions before serving if using.