This Thanksgiving classic is completely homemade - meaning there are no cans or packages of fried onions to open! It's a bit involved, so make sure to plan ahead when making it.
Add the onions to a medium mixing bowl and pour over the buttermilk. Stir to ensure the onions are coated entirely.
In a shallow dish, whisk together 100g (¾ cup) of flour, the salt, baking soda, garlic powder and cayenne pepper. Lift the onions from the buttermilk, allowing the excess to drip off, and add them to the flour mixture. Toss to ensure the onions are well coated in the flour.
Line a baking sheet with paper towels and set a wire rack over it. Add the oil to a Dutch oven or a large cast iron skillet (I prefer the latter) and bring to 175°C/350°F. Add the onions and fry until golden brown and crisp, about 3-5 minutes. You may need to work in batches. Remove from the oil and set on the prepared rack. Season generously with salt. Set aside.
Bring a large pot of generously salted water to a rolling boil. Fill a large bowl with equal parts ice and cool water. Add the green beans to the boiling water and cook for 90 seconds. Drain the beans and immediately add them to the ice bath to halt the cooking. Set aside.
Preheat oven to 175°C/350°F. In a large saucepan or Dutch oven, melt the butter along with 2 tbsp of the onion frying oil over medium heat. Once shimmering, add the mushrooms and a generous pinch of salt. Cook, stirring occasionally, until the mushrooms have released their moisture, are greatly reduced in size and are golden brown - about 10 minutes.
Add the shallot along with a generous pinch of salt. Cook until tender - about 2 minutes. Add the minced garlic and cook until fragrant - only 30 seconds longer.
Sprinkle over the remaining flour and stir to combine in the mushroom mixture. Cook, stirring constantly, for about 2 more minutes - until the flour takes on a toasty aroma.
Deglaze the pan with the white wine, scraping up any browned bits on the bottom. Cook until the wine has reduced and the strong alcoholic smell has cooked off - a minute or two.
Pour over the vegetable stock, milk and soy sauce. Bring to a simmer and allow the mixture to thicken slightly - about 5 minutes.
Drain the green beans (the ice should have melted by now) and stir them into the mushroom mixture. Pour the entire mixture into a 20cm (8in) square baking dish.
Top the green bean mixture with the fried onions. Move to the oven and bake until very golden and bubbling - about 20 minutes. Allow to cool for about 5 minutes before serving.
Notes
Use the weight measurement of salt rather than the volumetric measurement if at all possible. Different brands and types of salt can vary, so going by weight will be the most consistent option.