Step 1

Melt butter in a medium-size skillet over medium-high heat, then add mushrooms and cook for about 3 minutes on each side, or until golden and slightly crisped. Press down with a spatula to maximize browning.

Step 2

Add scallions to pan and cook for a minute, until wilted and slightly charred. Add salt and some black pepper to taste.

Step 3

Pour miso mixture over mushrooms, and turn over until evenly coated and absorbed.

Step 4

Toast buns, then assemble burger: First, place bottom half of each bun on a plate. Sprinkle each with ¼ cup of grated Swiss cheese. Place hot mushrooms and scallions on top of cheese layer. Top with sliced red onion and arugula.