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1
For the topping: Preheat oven to 500 degrees. Mix 2 tablespoons flour, 2 tablespoons breadcrumbs, and 1/2 tablespoon salt, and toss with half of the thinly sliced onions (1 onion) to coat. Liberally spray cookie sheet with cooking spray and spread onions on top. Spray onions with more cooking spray, and bake in oven for about 30 minutes, tossing onions every 10 minutes, until crispy. When they are done, lower the oven temperature to 400 and set the onions aside. In a small bowl, combine 3 tablespoons melted butter with remaining panko breadcrumbs (2 tablespoons) and set aside.
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2
Cook green beans in boiling water for 2 minutes. Drain, and plunge directly into ice water to stop the cooking process. Set green beans aside.
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3
Add remaining 3 tablespoons butter to large cast iron skillet over medium-low heat. Add the rest of the sliced onion (1 onion), salt and pepper, and saute in the butter until caramelized and reduced, about 20 minutes.
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4
While the onions are cooking, heat half and half in a small pot over low heat. Add dried porcini mushrooms and steep in the warm half and half until they are re-hydrated, about 5 minutes. Remove the mushrooms and dice, then add back to the cream and remove from heat. Set aside.
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5
When the onions are caramelized, remove them from the pan and set aside. Add the diced fresh mushrooms and saute over medium heat until the mushrooms are browned and have released most of their liquid, 5-8 minutes.
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6
Add the onions back to the skillet with the mushrooms, add minced garlic and saute over medium heat, 2 minutes. Add remaining 2 tablespoons flour and saute another minute.
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7
Add the cup of stock and half and half with steeped porcini mushrooms to the skillet. Stir constantly for the first two minutes, and then cook over medium-low heat until liquid is reduced by half to a creamy sauce, 5-8 minutes. Add green beans, toss to coat evenly in sauce, and remove from heat.
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8
Top green beans in skillet with the crispy onions and the panko-butter mixture, and a drizzle of truffle oil (optional). If serving immediately, cook in 400 degree oven for about 15 minutes, until bubbling and golden on top. If making in advance, store in the fridge up to 1 day and cook in 400 degree oven until hot and bubbling, about 30 minutes, before serving.
