For the fried onions

1. To make things easier for cleanup, place the sliced onions in a large gallon plastic zipper bag. Pour in the buttermilk, and toss the onions around to make sure they’re nice and coated. Press as much air out of the bag as you can, and zip it up. Let the onions soak for about a half hour, up to an hour.
2. Meanwhile, make the dredge by combining the self-rising flour and sea salt in a medium bowl.
3. When you’re ready to fry, pour about 2 inches of oil in the bottom of a tall-sided pot. Make sure you use a pot with high sides because the oil will rise as the onions fry. Heat the oil to 375 degrees F.
4. Dredge the buttermilk soaked onions in the flour mixture until they’re all evenly coated, shaking off any excess. When the oil is ready, fry your onions in batches for about 3 minutes, then drain on paper towels. They will be very blond looking at first, but don’t worry, they’ll brown up in the oven later.
5. Set aside.

For the casserole

1. Wash your green beans thoroughly with cold water. Chop or snap off any stringy/woody ends.
2. Bring a large pot of water to boil. Add 1 tablespoon of fine sea salt to the boiling water.
3. Add the green beans, and blanch them for exactly 4 minutes. Drain and cover with ice cold water to stop them from cooking. Set aside.
4. Preheat oven to 400 degrees F.
5. In a large skillet or braising pan, heat 2 tablespoons of butter and 2 tablespoons of olive oil over medium heat. Sprinkle in the crushed red pepper flakes and herbs de provence. When the butter and oil start to bubble and foam up, add the mushrooms. Give them a generous pinch of kosher salt and black pepper, then arrange them into an even layer across the bottom of the pan. You want to make sure each mushroom has some kind of contact with the pan so it can caramelize. First, they will release water, then start to caramelize. Once they’re in an even layer, leave them alone for about 3 to 4 minutes. Toss them around so they’re flipped over, arrange in flat layer, and leave them alone again to caramelize. Do this until they’re evenly caramelized all over, this will take a total of 8 to 10 minutes.
6. Add the minced garlic, and gently stir to cook for about 30 seconds.
7. Add the fresh thyme and rosemary, and cook for 10 seconds while constantly stirring.
8. Pour in the balsamic vinegar and worcestershire sauce, while scraping up any brown bits on the bottom of the pan. Most of this liquid will evaporate almost immediately, so drizzle in just a little more olive oil if needed.
9. Dump in the flour and toss with the mushroom mixture until every mushroom is coated. Cook for 1 minute. No white streaks of flour should remain.
10. Pour in the chicken stock, while scraping up any bits on the bottom of the pan. Adjust the heat if necessary, reducing it to medium-low. Immediately pour in the half and half, stirring constantly to avoid any lumps. Season this mixture with dried mushroom powder (if using), kosher salt, black pepper, cayenne, nutmeg, onion powder, garlic powder, and smoked paprika. Taste it, and adjust seasoning as needed. Allow this mixture to come to a simmer and thicken, about 3 to 5 minutes. When the mixture is gravy-thick, add the zest of one lemon, and the parmigiano reggiano cheese. Turn off the heat, and adjust seasoning again if needed.
11. Carefully add the blanched green beans, arranging them in an even layer. Top with an even layer of the fried onions and place in a preheated oven for 25 minutes on the middle rack.
12. It’s done when the onions have browned even more and the mixture is nice and bubbly. Let it rest for 10 minutes before serving, it’s molten hot, and damn good.