Green beans with mushroom-madeira sauce

3 tablespoons butter
6 ounces shiitake mushrooms, stemmed and sliced
6 ounces oyster mushrooms, sliced
3/4 teaspoon dried thyme
3 tablespoons chopped shallots
1/2 cup Madeira
1 cup whipping cream
1 pound fresh green beans, trimmed
Vegetable oil (for deep-frying)
2 large leeks (white and pale green parts only), thinly sliced crosswise

1. Melt 2 tablespoons butter in heavy large skillet over medium-high heat. Add all mushrooms and thyme; sauté 5 minutes. Add 2 tablespoons shallots; sauté until mushrooms are tender, about 3 minutes. Add Madeira and simmer until almost all liquid evaporates, about 2 minutes. Add cream and simmer until slightly thickened, about 2 minutes. Set sauce aside. Cook beans in large pot of boiling salted water until just tender, 5 minutes. Drain. Transfer to bowl of ice water; cool. Drain. (Sauce and beans can be made 6 hours ahead. Cover separately; chill.)
2. Pour enough oil into large deep saucepan to reach depth of 4 inches. Heat oil to 350 F. Place 1/4 of leeks in small metal strainer. Lower strainer into oil; fry until golden, 40 seconds. Lift strainer from oil. Drain leeks on paper towels. Repeat with remaining leeks in 3 more batches. Season leeks with salt.
3. Melt 1 tablespoon butter in heavy large skillet over medium heat. Add beans and remaining 1 tablespoon shallots; toss to heat through. Season with salt and pepper. Place beans on platter.
4. Bring sauce to simmer. Spoon sauce over beans. Sprinkle with fried leeks.