In a medium saucepan cook onion in hot butter until tender; add rice. Cook and stir for 2 minutes more. Carefully stir in water, bouillon granules, and pepper. Bring to boiling; reduce heat. Simmer, covered, for 20 minutes (do not lift cover).
Remove saucepan from heat. If desired, stir in peas. Let stand, covered, for 5 minutes. Rice should be tender but slightly firm, and the mixture should be creamy. (If necessary, stir in a little water to reach desired consistency.) Stir in Parmesan cheese.