The butter melts of Srednya fire. Stir the mushrooms, seasoned with a little salt * and simmer until the liquid has evaporated (about 20 minutes). Add the flour and stir and a few minutes. Add about 1 cup broth, stirring vigorously until the flour or starch is dissolved in the broth. Then poured into the remaining broth and mix well. Season with pepper and thyme (optional). The temperature is reduced to medium-low, and the sauce is allowed to simmer until it thickens (about 30 minutes), stirring frequently. * As the broth is savory, may or may not put salt on the mushrooms and broth is more rare (in 1 liter of water to dissolve less bouillon cubes, rather than writing it needed for packaging of the broth).