Season the brisket liberally on all sides with salt and pepper. Heat a dutch oven or heavy frying pan over high heat. When the pan is hot, add the avocado oil. When the oil is shimmering and spreads easily around the pan, add the meat and sear on one side for 3-4 minutes. Do not move the meat around. Flip and repeat. Transfer the brisket to the pan you’ll be baking it in (if you’re using a dutch oven, set the meat aside and you’ll return it to the dutch oven soon). In the same pan you seared the meat, fry the sliced leeks in the olive oil. When soft, add the remaining ingredients and cook over medium heat for 20 minutes. Taste and adjust seasoning as needed. Pour the sauce over the brisket (if using a dutch oven, return brisket to the pot and spoon some of the sauce over the top). Cover the pan tightly and bake at 300°F (150°C) until the meat is fork tender—that means a fork can pierce the meat with no resistance. The time this takes will depend on the size and shape of your meat, as well as the kind of pan you bake it in. In a pyrex or ceramic pan, your meat may be ready in two hours. In a disposable foil pan it may well take three hours. Refrigerate overnight. Slice meat against the grain. Reheat in the sauce, either in the oven or on the stovetop, when ready to serve. NOTE: If you feel like you have a lot of sauce/gravy at the end, but the flavor is not strong enough, put the sauce (without the meat) back on the stove and cook uncovered so it reduces and the taste intensifies. Then pour back over the meat.