In a large, deep skillet or saute pan, combine the fish stock, wine, lemon, peppercorns, onion slice, bay leaf, thyme, salt and cayenne and bring to a gentle boil. Reduce the heat and simmer for 5 minutes. Add the fish skin side down, cover, and poach at a simmer until just cooked through, about 8 minutes.

While the fish is cooking, bring a medium pot of salted water to a boil. Add the fiddlehead ferns and blanch until just tender, about 1 1/2 minutes. Drain in a colander and shock in an ice bath. Drain on paper towels. In a large skillet, melt the butter over medium-high heat. Add the shallots and garlic, and cook for 1 minute. Add the mushrooms, salt, pepper and thyme, and cook, stirring, for 3 minutes. Add the wine and chicken stock, and simmer, stirring, until the liquid has almost evaporated, 4 minutes. Add the fiddlehead ferns and parsley and cook over high heat until warmed through, about 1 to 2 minutes.

Remove the fish from the poaching liquid and place on a serving platter. Spoon the fiddlehead fern-chaterelle mixture over the fish and serve immediately.