Place the corned beef brisket in a large saucepan or Dutch oven. Sprinkle with salt and pepper, scatter the sliced onions over the top, and cover with water.
Cover the pan tightly and bring to a boil. Reduce the heat to low and simmer for 1 1/2 hours.
Rinse and drain the sauerkraut; add to the corned beef with the vinegar and brown sugar.
Cover and cook for about 2 hours longer.
Grate the potato and add it to the brisket; cover and cook for 15 minutes longer.