Dissolve 1/2 teaspoon dashi powder in 1/2 cup water. Set it aside.
Heat oil in a deep frying pan or wok over medium high heat.
Cook onion until transparent, about 2-3 minutes.
Loosen the beef slices so that they are not all clumped together. Add beef into the pan and cook until beef starts to brown.
Add dashi/water mixture, butter, soy sauce, mirin, sake and sugar. Gently stir occasionally and cook for about 5 minutes or until liquid is reduced down and slightly thickened.
Fill rice in the bowls. Divide beef and cooked liquid over rice.
Add toppings as desired and serve immediately.