Melt the butter in a saucepan over medium heat. Add the onion; cook and stir until tender. Stir in the flour until blended, then gradually stir in the milk so that no lumps form. Season with salt, pepper, poultry seasoning and mustard. Cook over medium heat, stirring constantly until the mixture thickens. Remove from the heat and add 2/3 cup cheese and pimento, stirring until cheese melts. Add undrained mushrooms to cheese sauce.