Trim fat from pork, and cut tenderloin crosswise into 1-inch-thick pieces. Sprinkle with pepper. Coat a large nonstick skillet with cooking spray, and place over medium-high heat until hot. Add pork; cook 3 minutes on each side. Remove pork, reserving drippings in skillet. Set pork aside, and keep warm.
Add lemon juice and green onions to skillet; stir in spinach. Cook, stirring constantly, 3 minutes, or until thoroughly heated. Arrange spinach evenly on plates. Top evenly with pork.