Greek-style Cape salmon

1. Season fish and set aside. Bring enough salted water to cover celery to boil and poach (simmer) for 4 to 6 minutes. Remove from stove. Drain off water through a sieve and discard. Set celery (cut into 10 cm lengths) aside.
2. Heat olive oil and butter in a wide, shallow saucepan and sauté onion until translucent. Add celery and 125 ml (1/2 cup) water. Cover and simmer for 5 minutes.
3. Arrange fish and lemon peel on top of celery and season. Add enough water to half-cover fish. Simmer, covered, for 12 to 15 minutes or until fish is opaque and flakes easily. Remove from stove and cool for 5 to 6 minutes. Drain off stock into a bowl. Sprinkle fish with fennel.
4. Using a balloon whisk, blend egg yolks with cold water in a bowl. Add lemon juice and beat briskly until frothy. Transfer to a clean saucepan and place on stove over low heat.
5. Slowly add 90-105 ml (6-7 tbsp) of the reserved warm stock, beating constantly and rotating pan until sauce comes to boil. Remove from stove.
6. Pour mixture over fish. Shake gently, season and return to stove. Heat through over low heat until sauce starts to bubble. Serve warm, or at room temperature.