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. Let stand, covered, for 5 minutes. Rice should be tender but slightly firm, and the mixture should be creamy. Stir in cheeses. Garnish with chopped chives.