In a medium saucepan, bring water to a boil. Immerse string beans and cook for 7 to 10 minutes, depending on their size. They should be slightly crisp. Drain and place in ice water to stop the cooking. Drain well and place in a medium mixing bowl. Add the peppers, jicama and tomatoes. In a small mixing bowl, combine mustard, lemon juice and chives. Whisk to combine, slowly adding the olive oil until totally emulsified. Add salt and pepper and taste for seasoning. Combine dressing with vegetables and thoroughly toss. Taste for seasoning. Place in a serving bowl and refrigerate until ready to serve.