Wipe dry the pork leg well and score the skin into squares without cutting the meat.
Turn the leg, skin side down, and make a few incisions (about 10 holes) with a sharp pointed knife and press the sliced garlic and chopped sage into each hole. Season with salt and pepper.
Take a rectangular oven dish about 10cm deep, and in which the pork leg would fit well. Cover the bottom of the dish with the sliced onions, about one centimetre thick.
Place the leg, skin up, on top of the onions and brush with the oil. Sprinkle with coarse sea salt and massage the oil and salt well into the gaps between the scored skin.
Fill the dish with water until it is three quarters full. Cover with foil and seal well to minimize water vapor. Leave to set at room temperature for about an hour.
Once the oven has reached 230°C, place the dish on the middle shelf. Cook for one hour.
After one hour, remove the foil. Reduce oven temperature to 180°C and cook for another 1.5 hours. Add water if necessary.
For crackling, raise temperature to 245°C and cook for another 25 minutes or till skin puffs up.