Ham & Caramelized Onion Grilled Cheese with mustard

1 large onion , very thinly sliced 1/4 teaspoon sugar salt , pinch 4 teaspoons spicy brown mustard 3 teaspoons horseradish 2 teaspoons honey 8 slices light[...]

3 tablespoons butter or 3 tablespoons margarine , softened 1 medium sweet onion , sliced and separated into rings 1 teaspoon brown sugar 8 slices pumpernickel[...]