If you don't have a panini press, place sandwiches in a hot skillet, and press with a smaller heavy pan. Cook until bread is golden brown; turn and cook until other side is golden brown and cheese is melted.
Southern Living SEPTEMBER 2005
Sauté first 3 ingredients in a lightly greased skillet over medium-high heat 8 minutes or until onion is tender.
Spread 1 tablespoon horseradish sauce on each of 4 bread slices; top evenly with roast beef, cooked onion, and cheese. Top with remaining bread slices. Brush melted butter on both sides of sandwich.
Cook in a preheated panini press 2 to 3 minutes or until golden brown. Serve with remaining horseradish sauce.
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