Pulse the bread in the bowl of a food processor until finely chopped. Transfer to a bowl. Place the scallions, bell pepper, and goat cheese in the bowl of a food processor, and pulse until finely chopped and well combined. Transfer mixture to the bowl with the breadcrumbs, and stir. Stir in the cilantro, half the grated Monterey Jack, salt, and pepper.
Heat oven to 350 degrees; position rack in the
center of oven. Place mushroom caps,
cups facing upward, on a large cookie sheet.
Spoon a heaping teaspoon of stuffing mixture
into each cap. Bake until mushrooms
are tender and filling is hot, 15 to 20 minutes.
Remove mushrooms, turn oven to
broil, and position rack about 4 inches from
broiler. Sprinkle mushrooms with remaining
grated cheese, and broil until cheese is
golden, about 1 minute. Serve immediately.