1. Cook beans, covered, in a large pot of lightly salted boiling water over high heat 10 minutes or until crisp-tender.
2. Place dill and next 3 ingredients in a large heatproof glass bowl. Drain beans well, and add to dill mixture. Add olive oil, and toss well. Add tomato and onion, and toss gently. Cover and refrigerate up to 24 hours.
3. Remove beans from refrigerator, and let stand, covered, at room temperature 1 hour. Add 2 tablespoons lemon juice, and toss gently. Taste for seasoning, and add remaining lemon juice, if desired.