Cook orzo in boiling chicken broth in medium-size saucepan for 10 minutes or until tender. Drain excess liquid if necessary. Place orzo in large bowl. Stir 1 tablespoon olive oil into orzo.
Add tomato, mushrooms, cucumber, green pepper, green onion and parsley to the orzo; stir until ingredients are well combined.
Whisk together the remaining 4 tablespoons olive oil, the vinegar, garlic, salt and pepper in small bowl. Pour over orzo mixture; toss gently until well combined.
Serve the salad over a bed of spinach.