Mushroom Gravy

Ingredients -

2 tbsp Butter 
1 cup Mushrooms sliced 
1/2 cup Flour 
1 liter Beef stock 
1/2 tsp Salt 
1/2 tsp Pepper

Procedure -

Melt the butter and sauté the mushrooms over a moderate heat. Cook until juices have evaporated, and they've begun to brown. Stir in the flour, reduce the heat to medium, and cook until the flour starts to brown.
Whisk in the beef stock gradually. Bring to a boil, reduce to a simmer and cook for 20 minutes. Season to taste.