In a fish cooker or other utensil large enough to hold the fish, put all ingredients except the fish. Bring this court bouillon to a boil and simmer for ten minutes.
Place the fish on the rack of the fish cooker or put it in a cheesecloth sling. Lower the fish into the court bouillon and simmer for twenty minutes. Let the fish stand in the liquid until ready to serve.
When ready to serve, gently transfer the fish to a board. Discard the thyme and tarragon sprigs and the garlic; use the remaining vegetables for garnish. Serve with Sauce Rouille.