Fill a large pot with 2-inches of water and place a steamer inside. Bring water to a boil. Add string beans and cover for 5 minutes, or until beans are cooked throughout but still crisp to the bite. If you don't have a steamer, toss beans in a large pot of boiling water for 4 minutes.
Thoroughly drain water then return beans to the emptied pot. Let beans sit, uncovered, for 2 minutes so that any water coating them has evaporated. Pour out any accumulated water.
Melt butter in a saucepan over medium heat. Cook for 3 minutes, or until melted butter has turned golden brown (this adds a slightly nutty flavor). Remove from heat and add lemon juice and beans and toss thoroughly. Sprinkle with salt and pepper and toss again.