Heat oven to 350°F. Bake French loaf as directed on can. Cool completely.
Meanwhile, in 12-inch nonstick skillet, melt butter over medium-high heat. Add onion and salt; cook 10 to 12 minutes, stirring frequently, until deep golden brown.
Cut French loaf in half crosswise, then in half horizontally. Spread 1 tablespoon horseradish cream over cut sides of bottom of each loaf. Top each loaf with half each of the beef, onions, arugula and loaf tops.