Start by mixing the sauce ingredients in a bowl. Combine the soy sauce, toasted sesame oil, sake, mirin, sugar, and rice vinegar. Whisk them together until smooth. Set this bowl aside; the flavors will meld beautifully.

In a pan, heat the vegetable oil over medium heat. Once it’s hot, toss in the minced garlic and sliced onions. Sauté for a few minutes until the onions become translucent. Next, add the bell pepper and stir for another 3-4 minutes until they are slightly softened yet still crunchy.

Increase the heat to high. Add the sliced Kobe beef to the pan. Sear it for about 2-3 minutes. You want the edges to brown nicely while the center remains tender. Season it with salt and pepper during cooking.

Pour the prepared sauce over the beef and vegetables. Ensure every component is coated. Stir well while cooking for an additional 2-3 minutes until the sauce thickens slightly.

Once ready, take the pan off the heat. Serve the mixture hot. Top it with chopped green onions or chives for a fresh finish. The vibrant colors will tantalize your guests.