1 cup ketchup
1/4 cup brown sugar
1 teaspoon salt
1 tablespoon grated lemon zest
1 tablespoon vermouth
1 tablespoon prepared horseradish
10 cups frozen cooked potatoes, thawed
In a large bowl, mix ketchup, brown sugar, salt, lemon zest and vermouth. Cover and refrigerate for 3 hours or overnight.
Remove potatoes from the refrigerator, and cut into cubes. When potatoes are tender, transfer them to a large plastic bag. Place cubes in the bag with the ketchup mix, and squeeze lemon juice and vermouth mixture over. Seal and refrigerate overnight.
While the potatoes are still warm, mix the horseradish and brown sugar into the potatoes and seal. Mix in the remaining marinade mixture, until well blended.
Preheat oven to 375 degrees F (190 degrees C).
Place all the potatoes into a medium baking dish. Tuck edges of potato halves under sheet of foil, and roll up. Dust with remaining lemon juice and mustard. Place rolled potatoes in the baking dish. Bake, uncovered, in preheated oven for 1 hour. Some of the tops may crack while baking. Serve warm.