Finely chop garlic cloves on a cutting board and sprinkle 1 teaspoon of the salt on top. Using the flat side of a large knife, mash and grind the cloves until they turn into a paste (the salt creates friction that helps a paste form). Mix the garlic paste in a bowl with the remaining salt, pepper and lemon zest.
Vigorously rub half of this mixture all over the brisket.
Preheat oven to 325 degrees Fahrenheit.
In a large ovenproof cast-iron pot or other heavy pot with a lid, heat olive oil. Sear brisket on each side until nicely browned, but not so long that the garlic begins to burn. Remove the meat.
Add lemon juice and water to the pot and bring to a boil. Use a spoon to loosen the bits of meat and garlic stuck to the bottom.
Spread the remaining garlic paste evenly on top of the meat and place the brisket back in the pot. The liquid should cover the meat halfway, if it doesn’t, add a little more water.
Add the leeks to the liquid around the meat. Cover the pot tightly (if you don’t have a lid, use foil) and cook for 2 to 3 hours in the oven, or until the meat is very tender.
Remove the lid, sprinkle parsley all over, and cook for 15 more minutes without the lid.
The brisket can be eaten immediately but is also fantastic the next day. Make sure to slice the roast against the grain. When served, top with the sauce around the meat, more fresh parsley, lemon zest and sea salt to taste.