In a large saucepan of 1 1 /2 cups water, cook rice according to package instructions; set aside.
Heat dashi broth, onion, sake, mirin, soy sauce and sugar in a large stockpot or Dutch oven over medium high heat. Bring to a boil; reduce heat and simmer, about 8 minutes.
Continue cooking, stirring occasionally, until onions are tender, about 5 minutes.
Stir in beef until cooked through and tender, about 2-3 minutes; season with additional soy sauce, to taste.
Divide rice into bowls. Top with beef, eggs and pickled ginger, garnished with green onions and sesame seeds, if desired.
for the dashi broth
In a small saucepan over medium heat, combine water and kombu. Bring to a boil; reduce heat and simmer, stirring occasionally, about 10 minutes. Remove from heat.
Stir in bonito flakes; let stand 5 minutes. Strain through a fine-mesh sieve; discard solids.