Insert garlic slices in crevices in the lamb; rub the joint with a few tablespoons of olive oil and salt and pepper. Place half the amount of each herb into the base of an ovenproof dish; pour over the wine vinegar. Lay lamb on top of herbs.
Roast in the preheated oven for 1 hour, basting with pan juices every 20 minutes.
Remove the lamb from the oven and transfer to a oven baking tray. Grill each side until lightly browned, about 4 minutes per side. Rest for 10 minutes; strain the cooking juices from the roasting dish and skim off the fat.
In a small pan, combine the meat juices, the remaining chopped herbs, cumin and 1 tablespoon olive oil; whisk over a low heat, about 5 minutes.
To serve, slice the warm lamb and arrange on a platter. Add 1 tablespoon lemon juice and the broad beans to the sauce. Taste for seasoning and pour over the lamb. Serve immediately.