Cook the rice in a rice cooker (or however you'd usually prepare it).
In a large pan, heat the oil over medium heat + add a pinch of salt, then sauté the onions until soft. Turn and stir the onions until they're evenly browned, then remove from heat.
Cook the steak in the same pan and sauté until the red is mostly gone, then add the soy sauce + mirin. Reduce the heat + simmer for ~3 minutes.
You can add the onions back into the pan afterward and add water or cook off the liquid to adjust the sauce consistency to your preference!
Serve hot over rice!