Prepare a campfire and let it burn down to hot coals or prepare a grill for medium heat (375°F).
In a large bowl, combine the steak, shrimp, red onion, and corn.
Mix the oil, Old Bay, and garlic in a small bowl. Season well with salt and pepper.
Pour the seasoned oil over the steak and shrimp mixture and toss well to combine and thoroughly coat everything.
Arrange four double layers of aluminum foil on a flat surface. Divide the surf and turf mixture equally among the foil sheets.
Dot the butter over the mixture, dividing it evenly.
Fold up and seal each packet. Place foil packs on a grate over hot coals or on the grill grate.
Cook until the shrimp is pink and opaque throughout and the corn is bright yellow, 12 to 15 minutes. If you're cooking over the campfire, your cooking time will be very dependent on the heat of your coals and the distance the foil packs are from the heat.
Open each packet. Top with chopped fresh parsley, if using, and serve with lemon wedges, if desired.