In a shallow bowl large enough to hold the steak, combine the Port wine, red onion, garlic and olive oil. Stir to blend well. Lay the steak over the marinade and then turn it over. Wrap loosely and refrigerate overnight being sure to turn the steak over at least once. Remove the steak from the marinade and season with the salt and pepper. Grill the steak over high heat for 4-5 minutes per side.
Remove from the grill and slice on the diagonal to serve.