Panera Bread Steak & White Cheddar Panini

Name: Panera Bread Steak & White Cheddar Panini
Prep time: 5 min
Cook time: 10 min
Total time: 15 min
Cuisine: American
Ingredients: 1 cup sliced red onions ((thinly sliced)); 1/2 cup white vinegar; 1 tablespoon sugar; 1 1/2 teaspoons salt; 1 cup warm water; 1 loaf ciabatta bread; 2 tablespoons pickled red onions; 2 tablespoons horseradish sauce; 6 ounces sliced roast beef; 4 slices white Cheddar cheese; cooking spray for pan