In a medium saucepan cook the 1/2 cup onion and garlic in hot oil until tender but not brown. Carefully stir in beans, undrained tomatoes, salsa, salt, and cayenne pepper. Bring to boiling; reduce heat. Simmer, uncovered, for 15 minutes.
To serve, mound rice on serving plates; make a well in each mound. Spoon the black bean mixture into wells. If desired, sprinkle with shredded cheese and the 1/4 cup chopped onion.