Get your grill nice and hot for searing.
Meanwhile, bring your salmon out and let rest to come up to room temperature (20 minutes or so).
Sprinkle fish with salt and pepper.
Use a neutral, high heat oil (like grapeseed oil) to rub the grill grate so the fish won't stick.
Sear the salmon briefly, flesh side down, (about 2 minutes).
Carefully use a spatula/turner to separate fish from grill and flip so the skin side is down and cook another couple minutes.
Pour marinade over the seared salmon along the entire length of the filet spreading the onions and oregano down the center of the fish.
Continue cooking until salmon is cooked through another couple minutes depending on how thick the filet is.
Remove to a plate and serve with lemon wedges!