Sauté the ginger slices with oil for a minute. Add the daikon and black peppercorns and stir-fry for 2 minutes. Add the beef stock, sake, mirin, and soy sauce. Bring the soup to a boil and then reduce the heat to keep the pot at a simmer. Cook for 20 minutes or until the daikon starts softening.
Add the Napa cabbage, shiitake mushrooms, and tofu and bring to a boil. Simmer at medium heat until the Napa cabbage is cooked, about 10 minutes.
Season with sea salt to taste. Remove from the heat.
Drizzle the soup with toasted sesame oil and serve.