Bring a large pot of salted water to a rolling boil. Drop the trimmed green beans into the water and boil for exactly 3 to 4 minutes until vibrant green and crisp-tender.

Immediately plunge the beans into a large bowl of ice water for 2 minutes to halt the cooking process. Drain thoroughly and pat dry with paper towels.

In a wide skillet, heat the oil over medium heat. Toss thin shallot rings in flour and shake off excess. Fry shallots in batches for 3 to 4 minutes, stirring constantly until light golden brown.

Transfer immediately with a slotted spoon onto a paper towel-lined plate. They will crisp into crunchy golden ribbons as they cool. Sprinkle with a pinch of sea salt.

Preheat your oven to 375°F (190°C). Melt butter in a large skillet over medium-high heat. Add sliced cremini mushrooms and cook undisturbed for 4 minutes until deeply browned, then stir in garlic and thyme for 1 minute.

Sprinkle flour over mushrooms and stir continuously for 1 minute to cook out raw flour flavor. Slowly whisk in broth and heavy cream. Bring to a gentle simmer for 4 minutes until thick, luxurious, and coating the back of a spoon. Season with nutmeg, salt, and black pepper.

Fold blanched green beans into the mushroom sauce until completely enveloped. Transfer into a 9x13-inch or oval baking dish.

Bake uncovered at 375°F (190°C) for 18 to 20 minutes until bubbling around the edges. Top generously with the crispy fried shallots and bake for an additional 4 to 5 minutes until shallots are hot and fragrant.

Rest for 5 minutes before serving alongside your holiday feast.