Combine the water, kosher salt, brown sugar, and pink curing salt if using in a large pot. Bring to a simmer, stirring until the salt and sugar fully dissolve. Add the garlic, bay leaves, peppercorns, mustard seeds, coriander seeds, cloves, allspice, ginger, and cinnamon sticks. Simmer for 3–4 minutes to release the spices' aromatics into the liquid, then remove from the heat and cool completely to room temperature, then refrigerate until fully cold. Do not add the brisket to warm brine. Warm brine begins cooking the meat's surface unevenly and can encourage bacterial growth before the salt has a chance to properly penetrate. Bringing the spices to a brief simmer before cooling blooms their aromatic oils directly into the liquid, producing a considerably more fragrant brine than simply steeping the same spices cold would achieve.

Place the trimmed brisket in a large non-reactive container or a heavy-duty brining bag. Pour the fully cooled brine over the brisket, making sure it's completely submerged. If needed, weigh it down with a plate. Cover and refrigerate for 5–7 days, turning the brisket once a day so it cures evenly on all sides. The longer end of this range produces a more deeply and evenly cured result, particularly for a thicker brisket. This is the main turning point in the entire recipe. A brisket cured for only a day or two will taste more like a seasoned pot roast than genuine corned beef, with the cure not having had time to reach the centre of the meat. Turning the brisket daily matters because even fully submerged meat can settle unevenly against a container's sides — that daily turn ensures every surface spends roughly equal time in the most concentrated part of the brine.

Remove the brisket from the brine and discard the used brine. Rinse the brisket thoroughly under cold running water to remove excess surface salt and spice residue. Pat dry. If desired, soak the brisket in a bowl of fresh cold water for 30–60 minutes, changing the water once, to draw out some of the surface salt before cooking — this step is optional but helps if the final dish tastes too salty. Rinsing thoroughly at this stage matters more than it might seem, since a week of curing leaves a considerable amount of surface salt and spice clinging to the meat that would otherwise oversalt the cooking liquid before it's even added.

Place the rinsed brisket in a large, heavy pot. Add the fresh pickling spices — peppercorns, mustard seeds, coriander seeds, cloves, allspice, bay leaves, and cinnamon stick — along with the quartered onion and smashed garlic. Add water or beef stock until the brisket is fully covered by at least 2–3cm. Bring to a boil, then immediately reduce to a low, gentle simmer. Skim off any grey foam that rises to the surface in the first 15–20 minutes. Cover and simmer gently for 3 to 3½ hours, until the brisket is fork-tender. Add the carrot chunks during the final 45 minutes of cooking if serving them alongside. Do not let this boil hard at any point. A hard boil toughens the muscle fibres faster than the connective tissue can break down, producing dry, stringy meat instead of tender corned beef. Skimming the grey foam in the first phase removes coagulated proteins that would otherwise cloud the finished cooking liquid and settle as a scummy layer on the meat's surface — this early attentiveness pays off directly in how clean the finished broth looks and tastes.

Remove the brisket from the pot and transfer it to a cutting board. Tent loosely with foil and let it rest for 15–20 minutes before slicing. Reserve some of the cooking liquid if you want to moisten the sliced meat or use it as a base for other dishes. This rest allows the juices, which contracted toward the centre of the meat during the long simmer, to redistribute evenly back through the whole brisket — slicing immediately would release much of that moisture onto the board rather than keeping it in the meat.

Identify the direction the muscle fibres run across the brisket. Slice against that grain into slices about 6–8mm thick for serving on a plate, or thinner if the corned beef will go into sandwiches. Slicing against the grain shortens each individual muscle fibre in every piece, which is specifically what makes a naturally fibrous cut like brisket genuinely tender to bite through — cut the wrong direction, even perfectly cooked corned beef will taste noticeably chewier than it should.

Serve the sliced corned beef warm, with the cooked carrots and onion alongside if desired, and a little of the reserved cooking liquid spooned over for moisture. Corned beef holds its temperature and texture reasonably well for a short window after slicing, but it's at its most tender and juicy served promptly rather than left sitting on the board.