Melt the butter in a heavy skillet and sauté the onion until tender. Add the mushrooms and cook for three minutes.
Increase the heat, add the chicken livers, and cook quickly until browned on all sides. Add the sherry and stir to loosen the browned-on particles. Season with the marjoram and salt and pepper to taste.
Remove the pan from the heat and stir in the sour cream and parsley. Use half of the mixture to fill the omelet. Add enough hot gravy to the remaining chicken-liver mixture to make a sauce to serve over the omelet.