Gyudon Japanese Beef Bowl

Ingredients

• large white onion
• teaspoon vegetable oil
• water
• tablespoons soy sauce
• tablespoon brown sugar
• tablespoon mirin
• tablespoon sake
• pound beef ribeye steak, thinly sliced
• tablespoon sesame seeds, or to taste
• green onions, thinly sliced, or to taste
• teaspoons pickled ginger , or to taste
• sheet dried seaweed, cut into strips, or to taste