Saute the onion and celery in 4 tablespoons of butter over medium heat until the onion is clear and the vegetables are soft. Add the thyme and marjoram and stir to combine. Set aside.
Cook the chopped mushrooms in 4 tablespoons of butter until their liquid is released and the mushrooms start to brown.
Pour vegetable mixture into mushrooms and stir to combine. Add salt and pepper to taste.