Preheat the oven to 425 degrees F.
Spray a casserole dish with olive oil.
Place the green beans in a large microwave safe bowl. Add ¼ cup water, cover and microwave until tender crisp, 3-4 minutes.
Meanwhile, in a small bowl, mix together the sour cream, salt, pepper, garlic powder and parmesan.
Set aside.
Transfer the green beans mixture to the baking dish. Sprinkle with the gruyere cheese. Lightly spray with olive oil.
Heat the broiler on high. Place the baking dish in the center of the oven, and broil until cheese is melted, about 2 minutes.
Serve immediately.
