Combine first 8 ingredients in a small bowl, stirring well with a whisk. Pour 1/4 cup juice mixture into a large zip-top plastic bag. Add chicken to bag. Seal; let stand 5 minutes. Add oil to remaining juice mixture; stir well with a whisk.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Remove chicken from bag; discard marinade. Add chicken to pan; cook 4 minutes on each side or until done. Place 1 1/2 cups spinach on each of 4 plates. Divide chicken evenly among servings; top each serving with 1 tablespoon juice mixture.