Bring water and rice to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until rice is tender and liquid has been absorbed, 20 to 25 minutes.
Combine dashi, soy sauce, mirin, sugar, and sake in a large saucepan over medium heat. Bring to a boil. Add onion; simmer until softened, 6 to 8 minutes. Add beef; simmer until no longer pink, about 6 minutes more.
Divide hot rice equally between individual deep rice bowls. Top with the simmered beef mixture.