To roast the maitake, place one tablespoon of the olive oil into a medium-sized cast iron pan, tilting it to cover the bottom. Place the maitake into the pan, taking care to kept them as intact as possible. Drizzle the remaining tablespoon of oil evenly over the maitake and place into the oven to roast for about 25 minutes, turning over once through the roasting time.
To make the broth, heat the vegan butter and toasted sesame oil in a small saucepan over medium-low heat. Add in the shallot and celery and stir to coat. Allow them to sweat for about 12 minutes, stirring occasionally and lowering the heat if needed if any of the vegetables start to brown.
Turn the heat up to medium-high. After 2 minutes, deglaze the saucepan with the sherry. Allow to simmer for 3-4 minutes more, then add in the vegetable broth. Allow to simmer for a few minutes, then remove from the heat. Remove a bit of the broth and add the miso to it. Stir until smooth, then add it back to the saucepan.
By now, your maitake should be almost ready. It’s done when it smells buttery and is very slightly crisp around the edges. Divide the maitake into two bowls, spooning the desired amount of broth over the top. Garnish with minced scallions and sesame seeds, and serve immediately.