Cook chicken breasts in large skillet over medium heat for two minutes on each side. Add milk, chicken broth, and onions. Cook about five minutes, turning chicken occasionally, until onions are cooked.Stir in spinach. Cook four minutes, stirring occasionally, until spinach is completely wilted and juice of chicken is no longer pink. Remove chicken from skillet; keep warm. Increase heat to medium. Cook spinach mixture about four minutes or until liquid has almost evaporated. Stir in salt and black pepper. Serve chicken on top of spinach.