Trim ends off beans. Bring a large pot of salted water to a boil. Add beans, bring water back to a boil and slowly boil the beans uncovered, for 8 to 12 minutes, or until tender but still crunchy. Immediately pour into colander and run under cold water for 3 to 4 minutes to stop the cooking. Remove to paper towels and dry. They may be refrigerated overnight at this point.
Before serving bring to room temperature. Melt butter or margarine in large skillet. Add beans, lemon juice, onions, and cashews; cook over moderately high heat stirring and tossing gently, until heated through. Season to taste.