Instructions

1. For the onions, fill a heavy Dutch oven or cast iron pot with a few inches of cooking oil. Heat over medium-high until the temperature reaches 375°F. Line a sheet pan with paper towels.
2. In a medium-sized mixing bowl, combine the flour and beer to make a pancake-like batter (thick but still pourable), using more beer if needed. Toss the onions in the batter and mix to coat evenly. Add the onions, separating as necessary, to the hot oil and cook until fluffy and light golden brown. Remove onions using a spider or slotted spoon to the sheet pan. Season generously with salt and set aside. (Crispy onions can be made 2 days in advance; store in an airtight container.)
3. To make the casserole, preheat oven to 350°F. Bring a large pot of salted water to a boil. Add beans and cook until deep green and tender, about 10 - 15 minutes. (I prefer my beans cooked until very tender, so I lean towards the latter time, but if you prefer more crisp-tender beans, cook for about 10 minutes, or less. The beans do not soften more in the casserole.) Drain and rinse with cool water.
4. In a large Dutch oven or heavy pot, melt 2 tablespoons of butter on medium-high heat. Add the mushrooms and sauté until they start sweating liquid, about 4 minutes. Add the onion, garlic, and a pinch of salt. Continue cooking, stirring often, until softened, about 6 minutes. Add the wine and simmer until it evaporates, about 3 minutes. Remove the mushroom mixture to a separate bowl.
5. Combine the milk, cream, and chicken stock in a large glass bowl and microwave until hot, about 2-3 minutes (or bring to a simmer in a saucepan). Melt the remaining 4 tablespoons butter in the Dutch oven over medium-low heat. Add the flour and whisk until creamy (and "raw" taste cooked out), about 2 minutes. Add the hot milk mixture and continue cooking, whisking frequently, until thickened, about 15 minutes. Whisk in Worcestershire sauce, mustard powder, and a generous amount of salt and pepper.
6. Stir the mushroom mixture into the cream sauce and simmer, whisking frequently, for about 5 minutes for the flavors to marry. Taste and adjust the salt and pepper. (You want this to be on the salty side, because it will mellow once you add the green beans.)
7. Stir the green beans into the mushroom sauce and toss to combine. Pour the green bean mixture into a large, buttered casserole dish (mine holds 2 quarts). Bake uncovered for 15 minutes. Add the grated cheddar and crispy onions and bake uncovered until cheese is melted, about 15 more minutes. Let stand for 10 minutes before serving.