📖 Instructions

1. First things first, prep your green beans! If you're using fresh, snap off those stem ends and blanch them. Bring a large pot of salted water to a rolling boil. Drop the trimmed green beans in and cook for about 4-5 minutes, just until they're tender-crisp and bright green. Don't overcook 'em; we want a little bite! Immediately drain and plunge them into an ice bath to stop the cooking process and preserve that vibrant color. Once cooled, drain thoroughly and set aside. If you're using frozen, no need to blanch; just thaw them well and drain any excess water.
2. Now, for the glorious golden onions! Thinly slice your yellow onions – I'm talking paper-thin slices here for the best crispiness. A mandoline works wonders if you have one, but a sharp knife and a steady hand will do the trick. Separate the rings and pat them really dry with paper towels.
3. In a shallow dish, whisk together the flour, salt, pepper, and a pinch of paprika. In another shallow dish, pour your buttermilk. Dip the dry onion rings into the buttermilk, letting any excess drip off, then dredge them generously in the flour mixture, making sure they're fully coated.
4. Heat about 2 inches of vegetable oil in a large, heavy-bottomed pot or Dutch oven over medium-high heat until it reaches 350°F (175°C). You can test it by dropping a tiny piece of onion in; if it sizzles vigorously, it's ready. Carefully fry the coated onion rings in batches, without crowding the pot, for 3-5 minutes per batch, or until they are deeply golden brown and wonderfully crispy. Use a slotted spoon to transfer them to a paper towel-lined plate to drain, and sprinkle them with a little extra salt while they're still warm.
5. Time for the creamy mushroom sauce! In a large saucepan or Dutch oven, melt the butter over medium heat. Add the sliced mushrooms and cook, stirring occasionally, until they’ve released their liquid and are nicely browned, about 8-10 minutes. Stir in the minced garlic and cook for another minute until fragrant.
6. Sprinkle the flour over the mushrooms and garlic, stirring constantly for 1-2 minutes. This cooks out the raw flour taste and forms a roux, which will thicken our sauce. Slowly whisk in the chicken broth, then the milk, making sure to whisk away any lumps until the sauce is smooth. Bring it to a gentle simmer, stirring frequently, until the sauce thickens to your desired consistency. It should be rich and creamy, coating the back of a spoon.
7. Stir in the heavy cream, dijon mustard, a dash of Worcestershire sauce, and season generously with salt and black pepper to taste. Taste and adjust; you want this sauce to be flavorful and seasoned well! Remove from heat.
8. Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish (or a similar size).
9. Combine the blanched green beans with the creamy mushroom sauce in a large bowl. Stir gently to make sure every bean is coated in that delicious goodness.
10. Pour the green bean and sauce mixture into your prepared baking dish, spreading it evenly. Give it a gentle shake to settle everything.
11. Bake, uncovered, for 20-25 minutes, or until the casserole is bubbly around the edges and heated through. You’ll see that wonderful creamy sauce bubbling up!
12. Remove the casserole from the oven. Now for the grand finale! Carefully spread the crispy golden fried onions evenly over the top of the hot casserole. Pop it back into the oven for another 5-10 minutes, just until the onions are heated through and regain their peak crispness. Keep an eye on them so they don't burn!
13. Let it rest for a few minutes after removing from the oven before serving. This allows the sauce to set slightly. Serve hot and watch it disappear!