Slice the onion into long, thin strips. Thinly slice the green onion.
In a pan without heat, whisk together dashi, mirin, soy sauce, sake, and sugar until combined.
Add onions and meat to pan.
Begin to cook over medium heat and cover with a lid.
When the meat is no longer red, reduce heat and simmer for about 3 more minutes.
In another frying pan, cook an egg sunny side up.
Serve gyudon over rice, top with egg, green onions, and shichimi togarashi.