The broth adds so much flavor and turns into a wonderful sauce to serve with the fish. It is best made with wine, but you can use stock as well.This recipe appeared on the Joy of Kosher web site and is from Tamar Genger
Heat the olive oil in a large pot or dutch oven. Add the onions, shallots, fennel, leek, garlic and ginger, saute on medium heat for about 2 to 3 minutes. Stir in the flour and cook until it is fully mixed.
Add the wine, bay leaf, thyme, salt and pepper to taste. The water and 2 tablespoons of the parsley. Bring to a boil and simmer for about 20 minutes. Place the frozen loaf of fish, unwrapped into the pot, bring to a boil and then lower to a simmer. Simmer uncovered 1 1/2 to 2 hours, turning occasionally.
Transfer fish carefully to a large platter and slice. Pull out the onions from the pot either by straining it or using a slotted spoon, discard. Heat liquid over high heat to reduce the liquid by half, it should take about 20 minutes. Pour over the fish and refrigerate for at least 2 hours.
Serve cold sprinkled with the rest of the parsley and a side of horseradish.
I found that after it had simmered for about 1 1/2 hours the sauce had already reduced by half so I didn't reduce it further. Also, I did not use the ginger.