Heat the butter over medium heat in a saucepan until it foams. Stir in the mushrooms and season with salt.
Simmer until the liquid evaporates, about 20 minutes.
Stir in the flour, cooking and stirring for about 5 min then stir about 1 cup of veg stock, then pour in the remaining stock and mix thoroughly. Season with salt and pepper.
Reduce heat to low and simmer until thickened, about 30 minutes, stirring often.