In a large soup pot, saute shallots or onions and garlic until translucent. Add the remaining vegetables and saute for 5 minutes. Add chicken broth and just enough water to cover ingredients. Season with salt, pepper, and chervil. Bring to a boil. Lower heat, cover, and let simmer about 30 minutes, until the mixture is reduced a little. The leeks get sweeter as they cook; when they taste right, add the fish and simmer for another 30 minutes or so, until the fish flakes easily. The soup will continue to reduce to a thick consistency. Add cream and heat through, then serve while piping hot.