Preheat broiler. Combine garlic, scallions, parsley, Worcestershire sauce, mustard powder, and softened butter. Mix to a paste. Season with salt and pepper.
Remove stems from mushrooms. Sprinkle lemon juice over the "veins" of the mushrooms. Add 2 snails to each mushroom and top with seasoned butter. Broil for 5 minutes. Serve on a bed of watercress.