Melt the butter in a large saucepan over medium heat, then add the mushrooms, giving them a quick stir to coat in the butter. Let the mushrooms cook for about 5 minutes, or until soft.
Toss in the flour, stirring until it is absorbed, then add the chicken broth. Stir and continue to cook for a minute or two, until the mixture thickens somewhat. Stir in the Imperial Seasoning and cream, and season with salt and pepper to taste.