Arrange the mackerel in a large oiled saucepan or plat à sauter. In another saucepan bring the white wine to a boil, add all the other ingredients except the peppercorns, and simmer, covered, for about 15 minutes. Add the peppercorns, pour the boiling liquid over the mackerel, return it to a boil, and maintain at a bare simmer, covered, for 10 to 12 minutes. Transfer the fish to a non-metallic container (an earthenware gratin dish is a logical and attractive container for conserving and serving), discarding all or most of the parsley and fennel branches. Refrigerate, covered, after cooling. Serve chilled.