Melt 4 tablespoons butter in a large skillet over medium heat. Add the shallots and saute until translucent, about 3 minutes. Add the mushrooms, thyme and sage and saute until most of the liquid has evaporated, about 10 minutes. Stir in the flour and cook 5 more minutes. Next add the sherry and stir until thickened. Add the cream and simmer an additional 5 minutes. Set sauce aside. (Recipe can be made to this point up to 2 days before, refrigerated.)
Melt the remaining butter in a large skillet over high heat. Add the beans, toss to coat with the butter and saute until bright green, about 7 minutes. They'll still be crispy. Season with salt and pepper and place on a platter.
Bring the sauce to a simmer and spoon over the beans. Top with the frizzled leeks.
Heat a couple inches of oil to 350 degrees F in a large, heavy pot. To check the oil temperature, drop a bread cube into it. If the cube browns within 30 seconds the oil is hot enough.
Trim leek, slice in half lengthwise, and wash well under running water. Cut the root end off the leek and then cut each half lengthwise into long, thin strips.
Season flour with salt and pepper. Dredge leek strips in flour. Fry leeks for 1 to 2 minutes, or until crisp and golden. Drain on paper towels.
Recipe courtesy of Dan Smith and Steve McDonagh