Puree 10 garlic cloves, red wine vinegar, thyme, and 1/2 cup water in a blender until smooth.

Place the leg of lamb in a roasting pan.

Pour the marinade over the lamb, ensuring it's well coated.

Cover the roasting pan tightly.

Refrigerate the lamb overnight to allow the marinade to penetrate.

Preheat the oven to 325°F (163°C).

Spread the chopped onion and remaining 10 garlic cloves evenly under the leg of lamb in the roasting pan.

Pour dry white wine into the pan around the lamb.

Rub the lamb with extra-virgin olive oil.

Place the bay leaves on top of the lamb.

Season the lamb generously with salt and freshly ground pepper.

Roast the lamb in the preheated oven for approximately 4 1/2 hours, or until an instant-read thermometer inserted into the thickest part registers 160°F (71°C) for medium doneness.

If the pan starts to dry out during cooking, add 1/2 cup of water to maintain moisture.

Once cooked, transfer the leg of lamb to a carving board and let it rest for 15-20 minutes before carving.

While the lamb rests, prepare the mint sauce: in a small saucepan, combine the remaining 1/2 cup of water, white wine vinegar, finely chopped mint, butter, and a pinch of salt.

Bring the mixture to a boil over medium heat.

Reduce the heat and simmer gently for about 3 minutes, or until the broth slightly reduces and resembles mint tea.

Carve the rested leg of lamb into slices.

Serve the carved lamb immediately, drizzled generously with the warm British mint sauce.