In a shallow bowl, toss the sliced shallots with flour, salt, and pepper until lightly coated. Shake off any excess flour.

Heat vegetable oil in a medium saucepan or skillet over medium-high heat until it reaches 350°F. Working in 2–3 batches, fry the shallots for 3 to 4 minutes per batch until golden brown and crispy. Do not crowd the pan.

Remove with a slotted spoon and drain on a paper-towel-lined plate. Season immediately with salt. Set aside. They will crisp up further as they cool.

Bring a large pot of water to a boil and add 1 tablespoon of kosher salt. Add the green beans and blanch for 2 minutes — they should be bright green and just barely tender.

Immediately transfer the green beans to a bowl of ice water to stop cooking. Drain well and pat dry with paper towels. Set aside.

Preheat your oven to 375°F. Grease a 9x13-inch baking dish and set aside.

Melt butter in a large, deep skillet over medium heat. Add the diced shallots and cook, stirring occasionally, for 4 to 5 minutes until softened and translucent. Add garlic and thyme, cook for 1 minute more until fragrant.

Increase heat to medium-high. Add sliced cremini mushrooms in a single layer and cook undisturbed for 4 to 5 minutes until browned on one side. Stir and continue cooking for another 3 minutes until mushrooms are golden and most of their moisture has evaporated.

Pour in the dry sherry and cook, scraping up any browned bits from the bottom of the pan, for 1 to 2 minutes until the liquid is mostly absorbed.

Sprinkle flour over the mushroom mixture and stir constantly for 1 minute to cook out the raw flour taste.

Gradually pour in the chicken broth, stirring constantly to prevent lumps. Then pour in the heavy cream. Bring to a gentle simmer and cook, stirring frequently, for 5 to 6 minutes until the sauce thickens enough to coat the back of a spoon.

Stir in 3/4 cup of the shredded gruyère, nutmeg, salt, and pepper. Taste and adjust seasoning as needed. Remove from heat.

Add the blanched and dried green beans to the mushroom cream sauce and toss to coat completely. Transfer the mixture into the prepared baking dish and spread into an even layer.

Sprinkle the remaining 1/4 cup of gruyère evenly over the top. Cover the dish tightly with foil and bake for 20 minutes.

Remove the foil and scatter the crispy fried shallots evenly over the top. Return to the oven uncovered and bake for an additional 10 to 12 minutes until the edges are bubbling and the shallots are deep golden.

Let the casserole rest for 5 minutes before serving. Enjoy immediately for maximum shallot crispiness.