Start by thinly slicing the medium shallots and heating about 1 cup of vegetable oil in a deep skillet over medium heat. Once the oil shimmers, carefully add the shallots and fry until they turn golden brown, about 4-5 minutes, stirring occasionally to avoid burning. Once crispy, remove them with a slotted spoon and drain on paper towels, then season with Creole Cajun seasoning for an added kick.

Bring a large pot of salted water to a rolling boil and add the fresh green beans. Blanch the beans for about 3 minutes, ensuring they stay vibrant green and slightly crisp. After the time is up, quickly transfer the beans to an ice bath to stop the cooking process. Once cooled, drain and set them aside while you prepare the creamy sauce for your homemade green bean casserole.

In a medium saucepan, melt 2 tablespoons of unsalted butter over medium heat. Add 8 ounces of sliced baby Bella mushrooms and 2 minced garlic cloves, sautéing them until tender and fragrant, which should take around 5-7 minutes. Stir frequently to ensure even cooking and prevent sticking. This step builds the delicious umami flavor that forms the base of your creamy mushroom sauce.

Once the mushrooms are nicely sautéed, sprinkle in 1/4 cup of all-purpose flour and mix well to combine, cooking for about 1 minute. Gradually pour in 2 cups of chicken broth, stirring constantly to avoid lumps. Add in your seasonings like 1 tablespoon of Dijon mustard, 1 tablespoon of fresh lemon juice, and a pinch of dried thyme. Cook until the sauce thickens, which should take around 3-5 minutes, and then stir in 1 cup of heavy whipping cream for a luscious finish.

Preheat your oven to 375°F (190°C). In a large bowl, combine the blanched green beans with the creamy mushroom sauce, ensuring they're well-coated. Pour the mixture into a greased 9x13 inch casserole dish. For an extra layer of flavor, sprinkle half of your crispy shallots on top before placing it in the oven. Bake for about 20 minutes, or until the sauce is bubbling.

After 20 minutes, take the casserole out of the oven and carefully sprinkle the remaining crispy shallots over the top. Return the dish to the oven and bake for an additional 7-10 minutes, or until the shallots are a deep golden brown and crunchy. This final step will ensure your Homemade Green Bean Casserole is both creamy and delightfully crispy.

Remove the casserole from the oven and let it sit for a few minutes before serving. The dish should be bubbling with a golden top and creamy texture. Serve hot as a comforting side dish, and watch as everyone gathers around to relish the layers of flavor in this homemade delight!