Cook orzo pasta according to package directions; drain well.
In a large skillet heat butter over medium-high heat; stir in the mushrooms, leek or green onion, pepper, salt, and garlic. Cook, uncovered, for 5 minutes. Add water, dried marjoram (if using), and soup base. Reduce heat; cook until liquid is almost absorbed, about 6 minutes. Stir in fresh marjoram, if using.
Toss mushroom mixture with pasta. Sprinkle with Romano cheese, if desired.