Clean the leg well and soak it for 12 hours in water. Prepare a marinade for the meat by mixing the finely chopped onion, garlic and rosemary leaves. Mix in the tomato paste, salt, vinegar, black and cayenne pepper. You need to get quite a thick sauce.
Smear it well on the leg and leave it to sit for 6 hours in a large pot. After it marinates, make incisions along the skin, pour in the remaining marinade and wrap it in foil. Put it to bake in a preheated 392°F (200 °C) oven for about 1 hour and 30 minutes.
Then pull the tray out of the oven and using a sauce brush, remove the remnants of the garlic and onions from the marinade, because they can burn. Smear the roasted leg with a mixture of melted butter and 1 tablespoon balsamic vinegar.
You can also add 2 tablespoons of Worcestershire sauce. Transfer the leg to a new foil and bake for another 15 minutes.