Here is the recipe I usually use. It *does* call for butter, but I kind of want to experiment with it. I had a Chicken Marsala at a restaurant recently, and they had obviously used a bit of demi glace in there. Wonderful, velvety texture and depth of flavor.
Also, how about making your sauce up ahead, and keeping it in a thermos until time to serve?
4 tablespoons butter or margarine
1/2 c chopped onion
2 garlic cloves
1/2 pound mushrooms, sliced
3 tablespoons dry Marsala
2/3 cup beef stock
salt and pepper to taste
In a large frying pan, heat 2 tablespoons of the butter. Add onion and garlic and saute' until onion is tender. Add mushrooms and cook until they are lightly browned.
Stir in Marsala and beef stock. Bring to a boil, reduce heat, and simmer until liquid reduced by one-third. Whisk in remaining butter. Season with salt and pepper to taste.