In a large skillet or wok, add a drizzle of oil and heat over medium-high heat. Once hot, toss in the sliced onions and sauté until they become tender and transparent.

Add the thinly sliced beef to the skillet. As it begins to cook, continue to stir, ensuring it browns evenly. The aroma will start to fill your kitchen.

In a bowl, whisk together the soy sauce, mirin, sake, and sugar. Pour this mixture over the beef and onions in the skillet.

Stir everything to combine, allowing the sauce to simmer for about 5 minutes.

Serve the hot beef mixture over bowls of freshly made rice, optionally topping with pickled ginger and chopped green onions.