Gyudon Japanese Beef Bowl
1 large white onion 1 sheet dried seaweed, cut into strips, or to… 1 tablespoon brown sugar 1 tablespoon mirin 1 tablespoon sake 1 tablespoon sesame seeds, or to taste 1 teaspoon vegetable oil 2 green onions, thinly sliced, or to taste 2 tablespoons soy sauce 2 teaspoons pickled ginger (beni shoga), or t… ½ cup water ½ pound beef ribeye steak, thinly sliced