Rinse the brisket. Rinse the corned beef brisket under cold water (and trim excess fat if you want), then place it in a large pot or Dutch oven.
Add seasonings + liquid. Sprinkle in the spice packet (or add bay leaves, mustard seeds, and black pepper), add the quartered onion and optional garlic, then pour in enough water or beef broth to cover the brisket by about 1 inch.
Bring to a boil. Set to high heat and bring the pot to a boil.
Simmer low and slow. Reduce to low heat, cover, and simmer for 2.5–3.5 hours (about 45–50 minutes per pound), skimming off any foam with a slotted spoon, until the meat is fork-tender.
Add carrots + potatoes. Add the carrots and potatoes, cover, and simmer for 15–20 minutes until mostly tender.
Add cabbage last. Place the cabbage wedges on top of the vegetables, cover, and cook 12–15 minutes more (just until tender, not mushy).
Rest the meat. Transfer the brisket to a cutting board, cover loosely with a clean kitchen towel, and let it rest 10 minutes.
Slice and serve. Slice the corned beef against the grain, arrange on a serving platter with the vegetables, and serve with gluten-free mustard (and salt/pepper to taste).