Add 2 inches of oil to a wide, high-sided skillet or a large pot. Turn the burner to medium heat.

Thinly slice an onion lengthwise. (That means your knife should be lined up with the root and the tip.)

Separate the onion slices and place in a medium bowl. Cover with 1 and 1/2 cups buttermilk and stir to coat. Let the onions sit in the buttermilk for about 15 minutes, or up to an hour. (your oil should only take about 15 minutes to heat, so if you plan to soak longer than that, you can turn off the heat.)

In a pie plate or shallow bowl, combine 1 and 1/2 cups flour, 1 and 1/2 cups Panko breadcrumbs, 1 and 1/2 teaspoons kosher salt, a pinch of cayenne pepper, and 3 tablespoons chopped parsley.

When the oil is hot (it should be about 360 degrees) and the onions have soaked, drain the onions. Working in batches, dredge the onions in the flour mixture until well coated, then transfer to the hot oil. Fry for about 2-3 minutes, until lightly browned. They will continue to darken when you remove them, so be careful.

Transfer fried onions to a paper towel lined plate. Continue until all the onions are fried.

Meanwhile, bring a large pot of water to a boil over high heat. Add about a tablespoon of salt. It should taste like the ocean.

When the water is at a rolling boil, add all the green beans at once and cover with a lid right away. Set a timer for 6 minutes. Check the green beans with a fork and make sure they are crisp-tender, adding 1 minute if necessary. Don’t let them get mushy!

Once the green beans are done, drain immediately and rinse with cold water to stop the cooking process. At this point, if you are making this ahead, you can wrap the beans in paper towels and refrigerate for up to 2 days.

If you are making the casserole right away, blot the beans with paper towels to dry them a bit.

Preheat your oven to 375 degrees F.

Meanwhile, melt butter in a large skillet over medium-high heat. When the butter is hot, add the mushrooms and sprinkle with 1/2 teaspoon salt. Saute for about 7-8 minutes, until they are lightly browned and tender.

Add garlic and thyme, and sauté for 1 more minute until fragrant.

Add 2 and 1/2 tablespoons flour and a pinch of cayenne pepper, and stir to coat for about 1 minute.

Whisk in the half-and-half and chicken broth. Bring to a boil over high heat, then lower to a simmer. Whisk occasionally, making sure there are no lumps. After 5-7 minutes, the mixture should be slightly thickened and coat the back of a spoon.

Remove from heat and stir in 3/4 cup parmesan cheese and the green beans.

Transfer the mixture to a 9x13 inch pan (or any shallow 3 quart pan). At this point, you can cover the dish and refrigerate for up to 4 hours. (Leave the onions on the counter)

Bake the casserole for 15 minutes at 375, until bubbling. (If you made this ahead and refrigerated, it might take a few minutes longer)

Sprinkle the fried onions evenly on top of the casserole. Return to the oven and bake for another 5-10 minutes, until warmed through.