When the beef is tender, drop in the carrot and potato pieces. Simmer for 10 to 15 minutes.
Add the green beans and straw mushrooms, and simmer for another five minutes.
Calculate the remaining liquid in the pan. You should have about two cups. If the liquid is less than that, pour in the reserved mushroom liquid so that you have two cups of liquid in the pan (see notes after the recipe).
Stir in the Bechamel sauce, a quarter cup at a time.
Allow the white (cream) stew to come to a simmer before turning off the heat.
Taste and add more salt and pepper, if needed.