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.
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.
