To poach corned beef: Bring an 8-quart pot of water to a simmer. (If using a blanching pot, remove strainer for now.) If the brisket has a heavy layer of fat on top, trim it so only a thin layer remains. Carefully lower meat into simmering water, cover and return to a full simmer over medium heat. Reduce heat to low and simmer meat for 3 hours. Turn off heat, but keep pot covered and let meat sit in the water for an additional 30 minutes.

To glaze corned beef: Ladle out 1 cup of the poaching liquid from pot (or substitute orange or pineapple juice) and pour onto the bottom of a shallow baking sheet (nonreactive if using juice). Carefully remove beef from water and place (fat side up) on baking sheet. (Reserve remaining poaching liquid to cook potatoes.) In a small bowl, combine brown sugar with mustard and, if desired, preserves. Brush mixture liberally over the exposed areas of meat.

To roast: Meanwhile, preheat oven to 325°F. Roast corned beef until hot and the glaze has caramelized and is becoming crisp, about 45 minutes.

To boil potatoes: While corned beef roasts, bring poaching liquid back to a boil and, if using a blanching pot, insert strainer. Add potatoes to boiling water and cook (covered) until tender, but not mushy, 15 to 20 minutes at a strong boil.

To carve corned beef: Transfer glazed meat to a carving board and let settle (loosely covered with aluminum foil) for 10 minutes. Slice each brisket into 1/3-inch slices, going against the grain with a sharp knife. Cover meat with foil while you finish potatoes.

To finish potatoes and serve: Melt butter in a 10- to 12-inch deep-sided skillet. Just before serving, drain potatoes, add to skillet and shake the handle gently to swirl each potato in melted butter. Sprinkle with chopped parsley, freshly ground black pepper and a light application of coarse salt. Swirl once more. Carefully spoon each potato into a warmed serving bowl. Serve potatoes while hot with the sliced corned beef. Pass mustard sauce at the table.