Thaw fish, if frozen. Rinse fish. In a large skillet or fish poacher combine wine or broth, onion, salt, pepper, and oregano. Bring to boiling; add salmon. Reduce heat and simmer, covered, for 10 to 12 minutes or until fish flakes easily when tested with a fork. Use a slotted spatula to transfer salmon and onions from cooking liquid to a large bowl or shallow dish. Discard cooking liquid. Cover and refrigerate salmon and onions for 1 hour or until chilled.
Meanwhile prepare Olive and Onion Relish. Cut bread into quarters. Slice each quarter horizontally in half. Drizzle cut sides lightly with olive oil. Remove salmon from refrigerator. Cut salmon fillet into 4 equal pieces. Place a piece of salmon on bottom of bread. Top with Olive and Onion Relish. Add top piece of bread. Wrap securely in plastic wrap. Refrigerate up to 4 hours. Squeeze lemon wedges over salmon and relish before serving. Makes 4 sandwiches.
Olive and Onion Relish
In a small bowl combine reserved onion wedges (from poaching liquid); pitted green olives, halved; pitted kalamata olives, halved; snipped fresh flat leaf parsley; and nipped fresh oregano.