Rub pork with pepper; cut into 1-in. cubes. In a large nonstick skillet, heat oil over medium heat. Add pork; cook and stir until no longer pink, 8-10 minutes., Meanwhile, in a Dutch oven, bring water to a boil. Stir in orzo and salt; cook, uncovered, 8 minutes. Stir in spinach; cook until orzo is tender and spinach is wilted, 45-60 seconds longer. Drain., Add tomatoes to pork; heat through. Stir in orzo mixture and cheese.