Cooking the green beans in boiling water keeps them crisp and tasty. If the green beans you're using are super fresh, then you might want to cook them for less time.
In a medium saucepan cook beans, covered, in a small amount of boiling salted water for 5-10 minutes or until crisp-tender; drain.
Meanwhile, in a small saucepan melt butter over medium heat. Add walnuts and ginger; cook for 2-3 minutes or until nuts are toasted. Remove from heat; stir in lemon peel and lemon juice. Stir nut mixture into cooked beans.