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Preheat electric panini maker according to manufacturer’s directions. Meanwhile, mix beef stock, rye whiskey and 1 teaspoon of the sage in medium saucepan. Bring to simmer on medium heat. Set aside, keeping au jus warm while preparing sandwiches.
Mix mayonnaise, horseradish and remaining 1/2 teaspoon sage in small bowl. Lightly brush 1 side of each slice of bread with olive oil. Place bread, oiled side down, on a clean work surface. Spread top of each slice with 1 tablespoon of the sage mayonnaise. Top 4 of the bread slices with roast beef, red bell peppers and arugula. Top each with second slice of bread, oiled side up.
Place 2 sandwiches at a time on the preheated panini maker. Cook 3 to 5 minutes or until bread is crisp and dark golden brown. Cut each sandwich in half. Serve with au jus for dipping.