Place the corned beef in a large stock pot or dutch oven with enough water to submerge the corned beef. Toss into the pot the mustard seeds, peppercorns and 8 cloves of peeled garlic. Bring the water to a boil, then reduce to a simmer. Simmer for 3-3.5 hours, uncovered.
About 15 minutes before the beef is done cooking, heat a large pot over medium-high heat. Toss in 1 tbsp of butter, and add the sliced cabbage. Cook the cabbage over medium/medium-high heat for about 8 minutes, stiring frequently. At this point add 4 minced garlic cloves to the pot, and squeeze in the juice of half of a lemon. Stir and continue to cook until the cabbage begins to brown, then remove from heat.
When the meat is done cooking, transfer it from the water to a plate. Toss the quartered potatoes into the boiling corned beef water and cook until fork tender, about 10 minutes.
To pan sear the corned beef, heat a large skillet over medium-high heat. Add 1 tbsp of butter, and transfer the corned beef to the skillet. Sear on each side until browning and crispy, about 2 minutes per side.
Drain the potatoes (you can save the seasoned water for a stock if you like!). Slice the corned beef and serve with the lemon garlic cabbage and potatoes. Sprinkle with parsley and enjoy!