Bring a large pot of salted water to a rolling boil. Add trimmed green beans and boil for exactly 4 minutes until crisp-tender and bright emerald green. Immediately plunge into an ice bath for 2 minutes to halt cooking. Drain completely and pat bone-dry with clean kitchen towels.

Melt unsalted butter in a large deep skillet over medium-high heat. Add sliced cremini mushrooms in a single layer and sear undisturbed for 3 minutes until deep golden fond develops. Stir in minced garlic and fresh thyme leaves, cooking for 60 seconds until fragrant.

Sprinkle flour evenly over mushrooms. Cook and stir continuously for 60 seconds to toast the roux and cook out any raw starch flavor.

Gradually stream in chicken bone broth while whisking continuously, scraping up all browned mushroom fond from the skillet floor. Whisk in heavy cream and bring to a gentle simmer for 3 to 4 minutes until luscious and coats the back of a spoon.

Remove from heat. Whisk in grated parmesan cheese, nutmeg, sea salt, and black pepper until completely smooth. Fold in the dry blanched green beans and 1/2 cup of crispy fried onions.

Preheat oven to 375°F (190°C). Transfer green bean mixture into a greased 9x13-inch ceramic baking dish. Bake uncovered for 18 minutes until bubbling around the edges.

Scatter the remaining 1 cup of crispy fried onions evenly across the top. Return to the oven for an additional 5 minutes until deep golden brown and shatteringly crisp. Rest for 5 minutes before serving warm.