In a small saucepan, melt the butter over medium heat, whisk in the flour, and cook, whisking constantly, for about 2 minutes, or until the butter bubbles up and smells nutty. Whisk about half of the braising liquid into the butter mixture until smooth, and then bring to a simmer. Pour the butter mixture into the pot with the rest of the braising liquid, onions, and mushrooms and stir to mix. Add the cream and bring to a simmer. If the liquid is too thick, add broth, water, or more cream. If it is too thin, boil it down for a couple of minutes. The sauce shouldn't be too thick.