• !For the Onion Strings
• Slice the onion in half; place the flat half down on your cutting board and slice each half into thin rounds.
• Place in a baking dish and cover with buttermilk and soak for at least an hour.
• Combine dry ingredients and set aside.
• Heat oil in small fryer or large dutch oven to 375 degrees.
• Grab a handful of onions, throw into the flour mixture, tap to shake off excess, and PLUNGE into hot oil. Fry for a few minutes and remove as soon as golden brown.
• Repeat until onions are gone.
• Place in a baking dish and cover with buttermilk and soak for at least an hour.
• Combine dry ingredients and set aside.
• Heat oil in small fryer or large dutch oven to 375 degrees.
• Grab a handful of onions, throw into the flour mixture, tap to shake off excess, and PLUNGE into hot oil. Fry for a few minutes and remove as soon as golden brown.
• Repeat until onions are gone.

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 12-15 minutes (we like ours a bit crisp tender; cook longer if you prefer softer beans. Drain and rinse with cool water.
• In a large pot or skillet, melt 2 tablespoons of butter on medium-high heat. Add the onion and sauté until they start sweating liquid, about 4 minutes. Add the mushrooms, garlic, and a pinch of salt. Continue cooking, stirring often, until mushrooms have softened, about 5 minutes.
• Add the wine and simmer until it evaporates, about 3 minutes.
• Add the remaining 4 tablespoons butter to the mushroom and onion mixture and melt over medium-low heat. Add the flour and mix into the vegetable mixture, cook on low heat for about 2 minutes.
• Combine the milk, half and half, and chicken stock and stir to combine. Add the milk mixture and continue cooking, stirring frequently, until thickened, about 10 minutes. Mix in Worcestershire sauce, mustard powder, and a generous amount of salt and pepper.
• Simmer everything for about 5 minutes for the flavors to marry. Taste and adjust the salt and pepper. (You want this to be a bit on the salty side, because it will mellow once you add the green beans.)
• Stir the green beans into the mushroom sauce and toss to combine. Pour the green bean mixture into a large, buttered casserole dish (about 2 quarts). Bake uncovered for 15 minutes. Add the grated cheddar to the top and bake another 10 minutes or until the cheese melts.
• Top with the crispy onion strings and bake uncovered for 5 more minutes. Let stand for 10 minutes before serving.
• Serve with additional onion strings (if there are any!)