Begin by heating a skillet over medium heat and adding a generous amount of oil. Thinly slice the shallots and fry them in the hot oil until they turn golden brown and crispy, about 5–7 minutes. Remove the fried shallots from the pan and let them drain on paper towels.

Bring a large pot of water to a rolling boil, adding a pinch of salt. Blanch the trimmed green beans in the boiling water for about 3 minutes until bright green and slightly tender, then transfer to a bowl of ice water.

In a medium saucepan, melt 3 tablespoons of butter over medium heat. Add the minced garlic and sauté for 1 minute. Stir in the sliced mushrooms and cook until tender, then add flour, whisking in the vegetable broth gradually. Cook until thickened, about 5–7 minutes. Season with salt and pepper.

In a large mixing bowl, combine the blanched green beans and the creamy mushroom sauce. Mix until fully combined. Transfer the mixture to a greased baking dish.

Preheat your oven to 375°F. Sprinkle the crispy shallots over the top of the casserole and optionally grate some Parmesan cheese on top. Bake for about 30 minutes until bubbly and golden brown.