Preheat oven to 375°. In an extra-large skillet, melt butter over medium-high heat. Add mushrooms. Cook, tossing occasionally, until mushrooms have released their liquid, about 5 minutes. Add chopped shallots and garlic; cook, stirring, 2 minutes. Add madeira; cook, stirring, until almost evaporated. Add flour; stir to coat mushrooms; cook 1 minute. Add about half of broth, scraping up bits from bottom of pan. Add remaining broth and cream; bring to a simmer. Cook until sauce has thickened slightly, about 5 minutes. Season with kosher salt, pepper and nutmeg. Add green beans; stir to coat.