Wash the sweetbreads, then leave to soak for 2 hours, changing the water frequently.
Cook in lightly salted water with the onion, carrot, leek, celery, peppercorns, juniper berries and bay leaf for about 30 minutes until tender. Drain and remove skin from sweetbreads. Discard vegetables.
Trim and cut the runner beans diagonally into 2.5cm (1 in) lengths