In large skillet, heat oil over medium heat. Add onions and peppers; salt and pepper to taste. Sauté until softened and slightly browned.
Meanwhile, cut bread in half horizontally; then into 4 sections. In small bowl, combine mayonnaise and horseradish; salt and pepper to taste. Spread over bottom half of bread.
Add roast beef to skillet; sauté until just warmed; divide evenly among sandwiches. Top with peppers and onions, sliced cheese and top half of bread.
Spray a preheated Panini grill with cooking spray. Place sandwiches on grill; spray tops of sandwiches with additional cooking spray. Grill until bread is golden brown and cheese is melted (about 5 minutes).
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