Preheat a grill to medium high.
Sprinkle the fish fillets with salt and pepper. Divide the onions, peppers, and garlic into 6 portions. Place a mound of onions, peppers, and garlic in the center of a 12-by-24-inch sheet of aluminum foil. Drizzle with olive oil, and season with salt and pepper. Place a portion of the fish on top, sprinkle with 1 tablespoon parsley and add 1/4 cup of the wine. Fold the foil into a packet and seal it on all sides. Repeat with packets for the remaining fillets.
Place the packets on the grill, seam up, and cook until the fish is opaque, about 15 minutes. It is best to test the cook time with the one packet, as cooking time varies grill to grill. Carefully open the packets and serve with lemon wedges.