How to Make It
In a small bowl, cover the seaweed with the boiling water and let stand until pliable, about 30 minutes. Drain and chop the seaweed; transfer to a bowl. Stir in the butter and season with salt and pepper.
Preheat the oven to 425°. Arrange the mushrooms on a rimmed baking sheet in a single layer. Drizzle with the olive oil, season generously with salt and pepper and toss to coat. Dollop three-fourths of the seaweed butter over the mushrooms. Roast for about 30 minutes, basting occasionally, until tender, deeply golden and crispy in spots.
Scrape the mushrooms and crispy seaweed onto a serving platter and top with the chives. Serve with lemon wedges and pass the remaining seaweed butter at the table.