1. Heat a grill or grill pan over high heat. Pound chicken between two sheets of plastic until 1/2 inch thick. Remove plastic. Sprinkle chicken generously with salt and pepper. Lightly oil grill or grill pan; cook chicken until lightly browned and cooked through, turning halfway through cooking time,10 to 15 minutes on a hot grill pan. (Time will vary depending on the grill.) Set aside.

2. Toss the onion with olive oil, salt, and pepper, and place on grill until lightly charred and soft, 8 to 10 minutes. Set aside.

3. Slice baguette lengthwise, and spread both sides with tomato jam. Slice chicken into 1/4-inch strips. Layer one side of baguette with chicken, reserved onions, and the arugula. Top with other side of bread, tomato jam facing down. Slice into three six-inch sandwiches.