Preparation

• Prepare the onions for frying: In a bowl, combine the sliced red onions and the buttermilk. Toss to coat the onions, then place them in the fridge for at least 15 minutes. In another small bowl, whisk together flour, panko, and dried mixed herbs.
• Heat the oil for deep-frying: Fill a Dutch oven or other deep pot with about 2 inches of vegetable oil, and heat to 375°F. To check that the oil is hot enough, toss in a pinch of panko mixture; if it starts to bubble and fry, you’re ready to go.
• Deep-fry the onions: Working in handfuls, lift the onions from the buttermilk and toss them in the flour/panko mixture. Gently lower the coated onions into the hot oil and fry until deep golden brown. Lift the onions from the oil with a heat-proof slotted spoon and place on paper towels to drain.
• Preheat the oven and prepare a bowl: Preheat the oven to 375°F and set a strainer over your sink and prepare a bowl with ice water.
• Blanch the green beans: Bring a large pot of salted water to a boil. Add the green beans and boil for 5 to 6 minutes until just tender, but still slightly crisp. Drain and then shock the green beans in the bowl of cold water to stop the cooking process. When cool, drain again and set aside.
• Cook the mushrooms: Melt the butter in a large skillet and add in the mushrooms. Season with salt and cook until softened and tender. Add in the garlic and thyme.
• Make the creamy sauce: Sprinkle the flour over the mushrooms and stir until the mushrooms are coated. Stir in the half-and-half and the chicken broth. Bring the mixture to a simmer and allow the sauce to thicken. Stir in the Parmesan cheese and green beans.
• Assemble and bake the casserole: Transfer the creamy green bean and mushroom mixture to a casserole dish. Scatter the fried onions over top, and bake in the oven for 15 to 20 minutes, until the casserole is bubbling. Serve hot.