Peel and thinly slice the onion. Use a mandolin to get uniformly thin slices. You can either leave the onion rings whole are cut them in half to create strings. Transfer the sliced onions to a bowl and separate the layers. Pour the buttermilk over the onions, pressing down to fully submerge the onion. Let sit while the oil preheats.
Add 2 inches of vegetable oil to a deep, medium saucepan. Place over medium high heat and bring up to 350°F. Meanwhile, combine the flour, seasonings, salt, and pepper in a shallow dish. Stir to combine.
Line a baking sheet with a rack or paper towel and set aside. When ready to fry, lift a third of the onion slices out of the buttermilk, shaking off the excess. Add to the flour and toss to evenly coat. use your fingers break the onion strings apart and shake off any excess flour.
Carefully add the coated onion strings to the hot oil – the oil will bubble up once the onions are added. Use a slotted spoon to stir the onion strings occasionally. Fry the onions for about 5 to 7 minutes until the onion strings are golden brown.
Use a slotted spoon to transfer the crispy onions to the prepared baking sheet to drain. Sprinkle with kosher salt. Repeat two more times. The onions can be made ahead of time and will keep in an airtight container for 2 to 3 days at room temperature.
Roasted Green Beans with Creamy Mushrooms
Preheat the oven to 400°F. Trim the stem end from the green beans. Transfer to a baking sheet and drizzle with olive oil. Season with kosher salt and pepper and toss to coat. Roast for 20 to 25 minutes until tender.
Finely dice the shallot and mince the fresh garlic. Slice the mushrooms and set aside. Place a large skillet over medium heat and add a tablespoon each of olive oil and butter. Add the shallots and sauté until the shallots are softened and translucent, about 2 to 3 minutes. When softened, add the sliced cremini mushrooms. Season with some kosher salt and sauté until the mushrooms are tender, about 5 minutes.
When the mushrooms are fully cooked and beginning to brown on the edges, add the minced garlic and thyme. Cook for another minute until the garlic is fragrant. Add the chicken broth and heavy cream along with the Dijon mustard Worcestershire sauce, and pinch of kosher salt and pepper. Bring the sauce back up to a simmer and let cook on low for 2 to 3 minutes.
To thicken the sauce, make a slurry by combining the cornstarch and water in a small bowl or cup. Add the slurry to the sauce and quickly stir to combine. Simmer the sauce until it thickens.
To serve, arrange the green beans on a serving platter. Spoon the mushroom sauce over the beans and top with a pile of crispy onions. Enjoy!