This is an incredibly easy dish to prepare and it makes for a very wonderful dinner for two. Use a dry champagne for best flavor. Serve with baked potatoes, string beans, and the rest of the champagne!
Combine lobster tails, champagne, green onions, and salt in a skillet; bring to a boil. Reduce heat to medium-low, cover the skillet, and simmer until lobster is cooked through, about 15 minutes. Transfer lobster to a plate.
Stir cream in champagne sauce in skillet. Bring to a boil and cook until sauce is reduced to about 1/3 cup, 5 to 10 minutes.
Whisk butter into champagne sauce, 1 tablespoon at a time, until sauce is smooth. Cover the skillet and remove from heat.
Remove lobsters tails from shells and slice into medallions. Spoon champagne sauce over lobster medallions.