Peel and cut the onion into 1-inch thick wedges, setting aside. Rinse the corned beef brisket under cold water to remove excess salt from the curing process—this is crucial for preventing the dish from becoming too salty. Place the beef fat-side up in a large pot (at least 6-quart capacity). Add the onion wedges, lager, water, apple cider vinegar, pickling spices, sugar, and bay leaves directly to the pot with the beef. Bring to a boil over high heat, then reduce to a gentle simmer, cover with a lid, and let cook for 2 hours 15 minutes to 2 hours 45 minutes. The beef should be fork-tender when done.

While the beef is simmering, prepare your vegetables: peel and cut the carrots into 2-inch pieces, cut the savoy cabbage into 8 large wedges through the core (this keeps them intact during cooking), and cube the potatoes into 1.5-inch pieces. I like to cut the potatoes larger than you might think—this prevents them from becoming mushy and falling apart in the broth. Keep the vegetables separate until they're ready to be added.

Once the beef is tender (after 2 hours 15 minutes to 2 hours 45 minutes), carefully remove it from the pot and place it on a cutting board. Cover it loosely with foil to keep it warm and allow the meat to rest. Strain out the onion and bay leaves from the broth if you prefer a cleaner presentation, though I like leaving them in for extra flavor. Add the potatoes and carrots to the simmering broth and cook for 15 minutes until they begin to soften.

Add the cabbage wedges to the pot with the potatoes and carrots, nestling them gently into the broth. Simmer for 12 to 15 minutes more until the cabbage is tender but still holds its shape—overcooked cabbage becomes mushy and loses its appeal. The vegetables should all be fork-tender at this point. I find that cabbage cooks faster than you'd expect, so check it at the 12-minute mark rather than waiting the full 15 minutes.

Slice the rested corned beef against the grain into ¼-inch thick slices—slicing against the grain ensures each bite is tender rather than stringy. Arrange the beef slices on a serving platter or individual plates, then surround with the cooked vegetables from the pot. Ladle some of the cooking broth over everything to keep it moist and flavorful. Serve immediately while hot.