In a 4-quart pan, bring 3 cups water to a boil. Add rice and salt. Cover and reduce heat to simmer, then cook until almost tender, 15 minutes. Turn off heat and let stand, covered, for 5 minutes before fluffing with a fork.
In a small pan, bring soy sauce, sugar, sake, and 3/4 cup water to a boil. Remove from heat and reserve.
In a 10-inch frying pan over high heat, cook onion in oil until it begins to brown, 3 to 5 minutes. Add beef and continue stirring for 2 minutes, then add 1/2 cup of soy-sake sauce and stir until beef is cooked, 2 minutes longer.
Divide rice among 4 bowls. Top with beef, remaining sauce, and cilantro. Serve with ginger, if desired.
Note: Nutritional analysis is per serving.