Tear the butter lettuce into large pieces and arrange in a serving bowl.
Blanch the green beans in salted boiling water until they are cooked but still crisp, about 5 minutes. Drain and rinse with cold water to stop the cooking process. Drain again and sprinkle over the lettuce. Top with the grated carrot.
In a jar, shake together the olive oil, lemon juice, salt and mustard. Taste and adjust if necessary. Serve the salad, drizzle the dressing over and toss.