1. Tear four 36- by 18-inch sheets of heavy-duty foil; fold in half to make 18-inch squares.
2. In a bowl, combine the green beans, squash, and mustard; sprinkle with salt and pepper. Divide vegetables evenly among the four pieces of foil, placing in centers. Top each square with a slice of salmon.
3. Fold together foil and seal with double fold, leaving room for steam to build. Broil for 20 minutes.
4. After transferring to plates, sprinkle each slice with lemon (and parsley, if desired).