We roasted the mushrooms in the hot coals of a campfire, but you can also roast them on the grill.
Preheat grill to medium. (If you are using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 5 to 6 seconds.) Assemble 10 pieces of foil, each about 10 inches square. Toss mushrooms, garlic, and mint with oil, salt, and pepper. Divide mushroom mixture among pieces of foil (about 1/2 cup per packet). Drizzle with oil. Fold and crimp to enclose.
Place packets on grill rack or nestle them in hot coals. Roast until mushrooms and garlic are soft and golden brown, about 25 minutes. Drizzle with oil, and garnish with mint leaves and flowers.