Bring a wide, shallow pan of water to a simmer with the lemon slices, white wine, parsley stem, and black peppercorns. Add the fish and poach for six to eight minutes, depending on size. The fish should not be cooked so long that it begins to fall apart. Remove, drain, and keep warm.
Strain the poaching liquid and bring to a boil. Reduce by two-thirds.
Add the green peppercorns to the pan and return to a boil. Reduce until only about one tablespoon of liquid remains.
Add the cream and bring to a light boil. Reduce by about one- third. Season with salt to taste.
Spoon the sauce over the cooked fish and serve immediately.