In a saucepan of boiling salted water add the shellfish and cook for 1-2 minutes, or until just firm. Drain and pat dry In a bowl combine the seafood with the beans, fennel and onion. In a bowl whisk together the lemon juice, vinegar, mustard and salt and pepper to taste. Add the oil in a stream whisking, and whisk the mixture until it is combined well. Pour the dressing over the salad and toss to combine. Garnish with the herbs.