Heat about an inch of vegetable oil in a skillet over medium heat. Once shimmering, carefully add the sliced shallots and fry them until golden brown and crispy, about 3-4 minutes. Remove the fried shallots, letting them drain on a paper towel, and season with Creole Cajun seasoning while they’re still hot. Set aside to maintain their crunch.

In a large pot, bring salted water to a rolling boil and add the trimmed fresh green beans. Blanch them for approximately 3 minutes until bright green yet tender. Immediately transfer the beans to an ice water bath to halt cooking, preserving their vibrant color. Once cooled, drain the green beans and set aside for the casserole.

In the same skillet, melt the unsalted butter over medium heat. Add sliced baby Bella mushrooms and minced garlic, sauté for 5 minutes until the mushrooms are tender and have released their juices. Sprinkle in the all-purpose flour, stirring constantly for another minute, then slowly pour in the chicken broth, lemon juice, Dijon mustard, Worcestershire sauce, dried thyme, red pepper flakes, nutmeg, salt, and pepper, simmering for 2-3 minutes until slightly thickened.

Pour the creamy mushroom sauce over the blanched green beans in a large mixing bowl, stirring gently until every bean is well coated. Transfer the mixture into a greased casserole dish, ensuring an even layer. Sprinkle half of the freshly grated parmesan cheese over the top for an added depth of flavor.

Preheat your oven to 375°F (190°C). Bake the green bean casserole uncovered for 20 minutes, allowing it to bubble and meld the flavors. Once time is up, remove the dish from the oven, give it a gentle stir, then sprinkle the crispy fried shallots evenly on top. Return the casserole to the oven for an additional 7-10 minutes, until the shallots are perfectly golden and crispy.

Once baked to perfection, carefully take the Homemade Green Bean Casserole out of the oven. Wait a few minutes before serving to let it settle.