Toss onions and fennel with garlic, oregano, and 1 tablespoon of oil.
Sauté in a non-stick pan until just tender.
Remove and put aside, cover to keep warm.
Brush Corvina with remaining oil, season with salt and pepper, then sear in pan for three to four minutes on each side until opaque when tested with a fork.
Place fillets on serving plate.
Add fennel and onion back to pan and quicklysauté one minute to reheat.
Spread fennel mixture around fillets and serve immediately with lemon.