Preheat the oven to 300 degrees.
Rinse the corned beef well, and dry off the liquid. Rub it all over with the pepper, coriander, garlic, and pepper.
Fill a Dutch oven with two inches of water and then add the beef with the fat side facing up. Put the lid on and cook for three and a half hours. Take the lid off and cook an additional 30 minutes. Set aside two cups of half inch cubes of corned beef for the hash. Save the rest of the corned beef for another use.
Meanwhile, prick the potatoes all over and microwave on a plate for five minutes. Turn them over and microwave 5 minutes more. Peel and cut into half inch pieces.
In a cast iron skillet, melt two tablespoons of the butter and cook the onion until it softens a little.
Add the rest of the butter and the potatoes. Cook until the potatoes start to brown and crisp, about 15 minutes.
Add the corned beef, Worcestshire sauce, and thyme and cook three minutes and serve.