In a large sautée pan, melt the butter over medium heat. Add the onions and sauté for 8-10 minutes or until golden brown. Add the mushrooms and sauté for 5-6 minutes or until the mushrooms release their liquid and are softened.

Add the stock, if desired, bring to a summer, and cook 8-10 minutes, or until the liquid has almost evaporated. Season to taste with salt and pepper. Stir in the parsley and serve hot.