How to Make It
In a medium bowl, cover the seaweed with 2 cups of warm water and let stand at room temperature until softened, about 20 minutes; drain.
In a large saucepan, heat the olive oil over high heat until shimmering. Add the beef and season with salt and pepper. Cook, stirring occasionally, until lightly browned all over, 5 to 7 minutes. Add the garlic and cook, stirring, until very fragrant, 1 to 2 minutes. Add the drained seaweed and the sesame oil and cook over moderately high heat, stirring, until sizzling, about 3 minutes.
Add 3 quarts of water to the saucepan and bring to a boil over high heat. Stir in the fish sauce, soy sauce, mirin, 2 teaspoons of salt and 1 teaspoon of pepper. Simmer over low heat, stirring occasionally, until the beef is very tender, about 1 hour and 30 minutes. Season with salt and pepper. Ladle into bowls. Garnish with sliced scallions and toasted sesame seeds and serve.