1. Preheat CRUX Panini Grill. Meanwhile, mix together mayonnaise, horseradish and mustard. Spread over 4 slices of bread; layer smoked Cheddar slices, roast beef, caramelized onions and smoked mozzarella slices evenly over top. Cap with remaining bread; butter bread slices on the outside.
2. Wearing oven mitts, carefully open lid of panini grill; center 2 sandwiches on lower grill plate. Press handle several times during cooking time to press sandwich. Cook for about 5 minutes or until cheese melts and bread is toasted.
3. Carefully remove sandwiches with oven mitts. Repeat with remaining sandwiches.