Rinse cod fillets, pat dry, cut into bite-sized cubes and season with salt and pepper.
Peel onion and cut into strips. Rinse fennel bulb, remove hard stalk and cut into into small cubes. Heat butter in a pan and saute onion strips with fennel until soft. Arrange mixture in a round baking dish. Top with fish.
Separate eggs. Whisk egg yolks with cream. Beat egg whites until stiff and fold into egg yolks. Pour mixture into the baking dish and sprinkle with chopped chives. Shake the dish gently to evenly distribute the mixture. Bake in preheated oven at 200°C (approximately 400°F) for about 20-25 minutes or until golden. Garnish with chives and serve.