In a small bowl, combine the flour, smoked paprika, garlic powder, salt, and black pepper. Add the thinly sliced shallots and toss until evenly coated.

Heat the vegetable oil in a deep frying pan or Dutch oven over medium-high heat until it reaches 350°F (175°C).

Working in batches, carefully add a handful of coated shallots to the hot oil, ensuring not to overcrowd the pan. Fry for 2-4 minutes, stirring occasionally, until golden brown and crispy.

Using a slotted spoon, transfer the crispy shallots to a plate lined with paper towels to drain excess oil. Sprinkle lightly with a pinch of salt while still warm. Set aside.

Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Bring a large pot of salted water to a rolling boil. Add the trimmed green beans and blanch for 4-5 minutes, or until tender-crisp and bright green.

Drain the green beans immediately and plunge them into an ice bath to stop the cooking process and preserve their vibrant color. Once cooled, drain thoroughly and set aside.

In a large skillet or Dutch oven, melt the butter over medium heat. Add the sliced cremini mushrooms and cook for 5-7 minutes, stirring occasionally, until they have released their liquid and are nicely browned. Remove mushrooms from the pan and set aside.

Add the diced yellow onion to the same skillet and cook for 3-4 minutes until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onion and garlic, stirring constantly for 1-2 minutes to cook out the raw flour taste, creating a roux.

Gradually whisk in the chicken broth, ensuring no lumps form. Bring the mixture to a simmer, then whisk in the heavy cream, Dijon mustard, Worcestershire sauce, black pepper, and salt.

Continue to simmer, stirring occasionally, for 5-7 minutes, until the sauce thickens to your desired consistency. Stir in the cooked mushrooms and grated Parmesan cheese (if using). Taste and adjust seasoning as needed.

Add the blanched green beans to the creamy mushroom sauce and stir gently to combine everything evenly.

Pour the green bean mixture into the prepared 9x13 inch baking dish, spreading it into an even layer.

Bake in the preheated oven for 20-25 minutes, or until the casserole is bubbly around the edges and heated through.

Remove the casserole from the oven. Evenly sprinkle the reserved crispy shallot topping over the hot casserole.

Serve immediately and enjoy your homemade Green Bean Casserole!