Herbed Chicken and Arugula Panini

Sprinkle chicken on both sides with salt and pepper, then 1 tablespoon thyme. Heat 2 tablespoons olive oil in heavy large skillet over medium-high heat. Add chicken; sauté until golden and cooked through, about 2 minutes per side. Transfer chicken to plate. Add remaining 4 tablespoons oil and garlic to skillet; stir over medium heat 15 seconds. Add vinegar and remaining 1/2 tablespoon thyme; cook 15 seconds, scraping up browned bits. Return chicken to skillet and toss until heated through, about 1 minute. Arrange 1 bread slice on each of 4 plates. Top with chicken, onion, and arugula, then drizzle with vinaigrette from skillet. Top with remaining bread.