Clean a 3–4 lb sterlet and cut it into pieces. Wash in cold water and wipe dry with a clean napkin. Arrange in a single layer in a silver saucepan, failing that in a casserole or sauté pan. Add ¼ lb butter, salt, the juice from ½ lemon, and 2 glasses of champagne. Five minutes before serving, place on top of the stove to boil and immediately serve in the same pan.