In a small dry skillet over medium heat, toast mustard seeds, coriander seeds, and allspice berries for 1–2 minutes until fragrant, shaking the pan frequently to avoid burning. Remove from heat and add whole cloves, juniper berries (if using), and crushed red pepper. Let cool, then lightly crush with a mortar and pestle or the back of a spoon to open the flavors. Set aside.

Brine / Cure

Make the brine

In a saucepan combine 5 cups water, kosher salt, brown sugar, pink curing salt, the prepared pickling spice, whole black peppercorns, bay leaves, smashed garlic cloves, and apple cider vinegar. Bring to a simmer over medium heat, stirring until salt and sugar dissolve and spices bloom, 4–6 minutes. Do not boil for long—just heat enough to dissolve solids and extract flavors. Remove from heat and let cool to room temperature.

Adjust volume

Once the concentrated brine is cooled to room temperature, add the additional cold water (6 cups) so the total brine is cold. The brine must be fully cool before contacting the raw brisket to avoid partial cooking and food-safety issues.

Cure the brisket

Place trimmed brisket into a nonreactive container or heavy-duty resealable bag. Pour the cooled brine over the brisket, ensuring the meat is fully submerged; weigh it down with a plate if necessary. Refrigerate for 5–7 days, turning the meat once a day to ensure even curing. For a slightly stronger cure, cure up to 10 days, but 5–7 days is standard for a 4-lb brisket.

Cooking / Finishing

Prepare for simmering

After the curing period, remove brisket from brine, rinse thoroughly under cold running water to remove excess salt and surface spices, and pat dry with paper towels. Reserve the brine if you want to use a cup for cooking liquid flavor (optional) but discard remaining brine. Place rinsed brisket into a large pot. Add halved onion, carrots, celery, and enough fresh water or beef stock to cover the brisket by about 1 inch (approximately 6–8 cups). Add the optional additional pickling spice in a small cheesecloth sachet or tea infuser, if desired.

Simmer gently

Bring pot to a gentle simmer over medium-high heat. Reduce heat to low so the surface barely moves—maintain a gentle simmer. Skim any foam that accumulates for clarity. Cover and simmer gently until brisket is fork-tender, about 3–3.5 hours for a 4-lb brisket. Check after 2.5 hours: it should yield to a fork but not fall apart completely.

Finish and rest

When tender, remove brisket from the pot and transfer to a cutting board. Let rest 20 minutes, uncovered or lightly tented with foil. Meanwhile, strain and skim the cooking liquid to use as a light jus or discard. Taste the cooking vegetables and broth and salt/pepper lightly as desired. Slice corned beef across the grain into thin slices for serving.

Serving suggestions

Serve warm slices with cooked carrots, cabbage (optional—add wedges to the pot for last 20–30 minutes of cooking), boiled potatoes, or on rye bread with mustard. Use reserved cooking liquid to moisten slices if desired.