Preheat oven to 350°F.
Bring large pot of salted water to a boil over high heat. Add green beans or Brussels sprouts and cook until bright green, approx. 2 minutes for green beans or 4 minutes for Brussels sprouts. Drain beans or sprouts and plunge into bowl of ice water to cool. Drain beans or sprouts again, place in bowl and set aside.
In large saucepan over medium-high heat, combine half-and-half, seasoning mix and mushrooms and bring to boil. Cook, whisking constantly, until thickened, approx. 1 minute. Pour over beans or Brussels sprouts and stir to combine. Transfer mixture to a 13" x 9" baking dish and sprinkle fried onion topping evenly over top.
Bake until edges are bubbling, approx. 25 minutes. Transfer to wire rack and let rest for 5 minutes before serving.
Serves 8–10.

Williams Sonoma Green Bean Casserole Starter Mix