Preheat the oven to broil (about 500°F). Position the oven rack about 6–8 inches below the broiler.
Place the 4 large mushroom caps gill-side up in an oven-safe skillet or on a baking sheet.
Spoon 1 tablespoon of pizza sauce into each mushroom cap (use all 4 tablespoons).
Divide the 8–12 pepperoni slices evenly among the mushrooms (about 2–3 slices per cap) and place them on top of the sauce.
Sprinkle the 1/4 cup grated Monterey Jack cheese evenly over the pepperoni-topped mushrooms.
Broil the mushrooms for 8–10 minutes, watching closely, until the mushrooms have softened and the cheese is melted and beginning to bubble.
If the mushrooms are still firm after broiling, remove the skillet from the oven and cook it on the stovetop over medium heat for about 3–5 minutes to soften the mushrooms, watching so the cheese does not burn.
Remove from heat and let cool for a minute before serving.