Soak the sliced onions in the buttermilk for 15 minutes. Whisk the flour and 1 tsp salt in a bowl. Heat about an inch of oil to 350°F in a heavy pan.

Lift the onions out, dredge in the seasoned flour, shake off the excess, and fry in batches until deep golden, 2-3 minutes. Drain on paper towels and salt lightly. Set aside.

Bring a big pot of water to a boil and add 1 tbsp salt. Set up a large bowl of ice water right next to the stove.

Boil the green beans for exactly 4 minutes, then drain and dump them straight into the ice water. This stops the cooking cold and locks in that bright green, crisp-tender bite. Drain well and pat dry.

Melt the butter in a large skillet over medium-high heat. Add the mushrooms and cook, undisturbed at first, until browned and the liquid has cooked off, 6-8 minutes. Stir in the garlic for 30 seconds.

Sprinkle in the flour and stir for 1 minute to cook off the raw taste. Slowly whisk in the broth, then the milk. Add the soy sauce, pepper, and nutmeg. Simmer until thick enough to coat the back of a spoon, 3-4 minutes. Taste and salt as needed.

Heat the oven to 375°F. Fold the dry green beans into the mushroom sauce and spread in a buttered 9x13 baking dish. Top with about half the crispy onions.

Bake 20 minutes until bubbly. Scatter the rest of the onions over the top and bake 5-10 more minutes until golden. Let it rest 10 minutes before serving.