Green Bean Casserole with Fresh Beans

Preheat the oven to 350 degrees F (175 degrees C). Prepare an ice bath by filling a large bowl with ice and cold water. (5 minutes)

Bring a large pot of lightly salted water to a rolling boil. Add the green beans and cook, uncovered, until they are crisp-tender, about 6 minutes. Immediately drain the green beans in a colander and plunge them into the ice bath for several minutes to halt the cooking process. Drain well and transfer to a large bowl. (15 minutes)

In a large skillet, melt 3 tablespoons of butter over medium-high heat. Add the diced onions and sauté until translucent, approximately 5 minutes. Incorporate the sliced mushrooms and minced garlic; continue to sauté until the mushrooms are just cooked through, another 5 minutes. Combine this mixture with the green beans in the bowl. (15 minutes)

Place the remaining 5 tablespoons of butter in a 2-quart, microwave-safe bowl. Microwave on high in 30-second intervals until completely melted. (2 minutes)

Whisk the flour into the melted butter until a smooth paste forms; season generously with sea salt and freshly ground black pepper. Gradually add 1 cup of milk, whisking continuously until smooth and fully incorporated. Microwave for 2 minutes. Repeat this process, adding another cup of milk, whisking until smooth, and microwaving for 2 minutes. The roux mixture should thicken noticeably between additions. Finally, add the remaining 1 cup of milk and microwave for 2 minutes more, then whisk until perfectly smooth. (10 minutes)

Transfer the green bean mixture to a 9x13-inch baking pan; evenly distribute the thickened roux, stirring gently to combine. Season to taste with additional salt and pepper. (5 minutes)

Bake in the preheated oven until the casserole is warm and bubbly, about 30 minutes. Sprinkle the crispy fried onions evenly over the top and return to the oven for the final 5 minutes, until golden brown and fragrant. (35 minutes)