Turn oven on to 375°F. Place baking sheet in oven while preheating. Toast 5-7 minutes. Transfer to bowl and set aside. Place steaming basket in medium saucepan. Add water up to bottom edge of steamer. Bring water to boil, add green beans and cover.
Steam 4-5 minutes until tender.
Place cooked beans in large bowl, toss with butter, lemon juice, and lemon zest.
Season with salt and pepper.
Transfer beans to serving dish, sprinkle with toasted walnuts, and serve.