Prep the chicken. Preheat oven to 425°F. Line a rimmed sheet pan with parchment paper. Place chicken breasts in pan and coat with oil, salt, and pepper. Bake 20 to 24 minutes or until chicken is done (165°F). Top with cheese. Top chicken breasts with pesto and cheese. Bake 2 to 3 minutes longer or until cheese is melted. Plate and serve. Divide arugula and onion among serving plates. Top with chicken and tomatoes. Drizzle. In a small bowl, whisk together the remaining 1/2 cup olive oil and the vinegar; drizzle overall.