Preheat an oven to 350 degrees F, or a grill to high (if you are outside, prepare a campfire).
Cut a large sheet of foil and drizzle with some oil. Arrange the Brussels sprouts in a flat, single layer over half the sheet of foil. Drizzle the Brussels sprouts with more oil and top with the garlic, onion, lemon juice and a splash of vermouth. Sprinkle with salt and pepper. Fold the foil in half over the Brussels sprouts and fold the edges to seal and form a pouch. If the sprouts are crowded, make two pouches.
Cook the pouch in the oven or on a grill or campfire until cooked through but not mushy, about 20 minutes. To serve, cut open the foil pouch tableside and finish with a sprinkle of parsley.