Melt the butter and add the flour when it's bubbling. Whisk continually to cook the flour and not allow the two to burn. Slowly pour in the half and half and whisk to incorporate and to prevent sticking and burning. When it's starting to thicken add the baking powder and incorporate. Now add your lemon juice , lemon peel, chives, and black pepper. Whisk. Turn the heat off. Transfer to a serving dish and cover the top with wax paper (to prevent a skin) until the fish is cooked through. Use liberally!
Chop the carrots and onions and toss into a dish along with the bay leaf, lemon, and peppercorns. Simmer to create a very mild stock. Usually about 15 to 20 minutes. Add the fish and poach slowly, basting the top with the simmering water if necessary.