01 - Preheat oven to 425°F. Line a baking sheet with parchment paper.
02 - Melt butter in a small saucepan over medium heat, swirling frequently until it foams and turns golden brown with a nutty aroma, about 3 to 4 minutes. Remove from heat immediately.
03 - Whisk miso paste, olive oil, garlic, soy sauce, rice vinegar, and black pepper into the brown butter until smooth.
04 - Place mushrooms in a large bowl. Pour the brown butter miso sauce over and toss until evenly coated.
05 - Spread mushrooms in a single layer on the prepared baking sheet.
06 - Roast for 25 to 30 minutes, stirring halfway through, until mushrooms are deeply browned and caramelized at the edges.
07 - Transfer mushrooms to a serving dish. Sprinkle with fresh chives or parsley and toasted sesame seeds if using. Serve warm.