Heat about ½ inch of vegetable oil in a medium saucepan over medium heat. Add finely sliced shallots and fry until golden brown, approximately 15-20 minutes. Transfer to paper towel to drain and sprinkle with Creole Cajun seasoning.

In a large pot, bring salted water to a boil, then add the trimmed fresh green beans. Blanch for 3 minutes until tender yet bright green. Transfer beans to an ice bath, drain well, and set aside.

In a large skillet, melt unsalted butter over medium heat. Add chopped baby Bella mushrooms and minced garlic, sauté until tender, about 5-7 minutes. Stir in all-purpose flour and cook for 1 minute, then whisk in chicken broth while scraping up browned bits. Simmer and add lemon juice, Dijon mustard, Worcestershire sauce, thyme, red pepper flakes, nutmeg, salt, pepper, and heavy cream. Simmer for an additional 5 minutes until slightly thickened.

Pour the creamy mushroom sauce over the blanched green beans in a large casserole dish. Stir to coat all beans. Sprinkle half of the fresh Parmesan cheese over the top and mix well.

Preheat the oven to 375°F (190°C). Cover the casserole dish with aluminum foil and bake for 20 minutes. Remove the foil, sprinkle with crispy fried shallots and remaining Parmesan cheese, then bake for an additional 7-10 minutes or until bubbly and golden brown.

Let the casserole rest for a few minutes before serving hot as a side dish during holiday gatherings.