Sauté sliced onion in a lightly greased nonstick skillet over medium-high heat 6 minutes or until tender. Remove from heat, and stir in 2 Tbsp. balsamic vinegar. Let mixture stand 5 minutes.
Cut bread into 4 pieces. Cut each piece in half lengthwise, cutting to but not through opposite side. Place bread pieces, cut sides up, on a wire rack in a broiler pan.
Broil 6 inches from heat 3 to 4 minutes or until lightly toasted.
Spread cut sides of bread evenly with mustard. Stir together meat and gravy; spoon evenly onto bread. Sprinkle with cheese.
Broil 6 inches from heat 4 to 6 minutes or until cheese is melted. Toss together onion mixture and arugula; top sandwiches with mixture.