Place the mackerel in a roasting pan and pour in the white wine and court bouillon. Add the onion, carrots, lemon, peppercorns, and cloves to the pan and bring to a boil. Reduce heat, cover the pan with aluminum foil, and simmer for 12-15 minutes. Remove pan from heat and let fish cool in the poaching liquid. To serve, transfer the fish to a deep serving platter and strain the cooking liquid. Sprinkle the onion, carrot, and lemon slices from the cooking liquid over the fish. Reduce the cooking liquid by one-third and pour it over the fish.
