Set your oven temperature to 400°F (205°C) and prepare a baking sheet by covering it with parchment paper to keep things tidy.
Whisk together the soy sauce, brown sugar, minced garlic, grated ginger, sesame oil, rice vinegar, and black pepper in a bowl, adding the red pepper flakes if you want a little spice.
Toss the thin ribeye slices in the marinade, making sure each piece is coated well, then let it rest for about 15 minutes while you slice the onion.
Arrange the marinated beef evenly across the baking sheet without crowding, then sprinkle the sliced onion over the top.
Place the pan in the oven and roast the beef for 15 to 20 minutes until it develops a nice, caramelized edge.
Once out of the oven, sprinkle the dish with the chopped green onions and sesame seeds to add fresh flavor and texture.