4 to 6 servings
For the broth
3 large scallions, trimmed
1 handful dried shiitake mushrooms, crushed (about 1/2 cup)
1/4 cup crumbled dried seaweed, such as kombu or dulse (optional)
1 (2,5 cm) chunk of ginger, smashed and unpeeled
2 whole, unpeeled garlic cloves
Stems or trimmings from fresh shiitake mushrooms (optional)
1 teaspoon salt
For the soup
450 g dried udon noodles (or use fresh or frozen, cooked udon)
2 tablespoons butter
1 cup chopped leeks, white and pale green parts only
225 g fresh shiitake mushrooms, stems removed, caps sliced in 2 and a half cm strips (save trimmings for broth)
1 teaspoon minced ginger
2 cloves garlic, minced
1/4 cup mirin or dry sherry
1/2 teaspoon sesame oil
2 tablespoons soy sauce
2 cups chopped bok choy
1/2 cup red or white miso
1 cup soft or medium tofu, cut in small cubes
4 scallions, finely chopped, for garnish
Shichimi togarashi or crushed red-pepper flakes, for garnish
Make the broth: In a large pot, bring 8 cups water to a boil over high heat. Add scallion, dried mushrooms, seaweed (if using), ginger, garlic, fresh mushroom trimmings and salt. Reduce heat to a gentle simmer and cook for 20 minutes. Strain and set aside.
Meanwhile, in a separate pot, cook udon noodles until al dente, then drain, rinse well with cold water, and keep at room temperature.
In a soup pot, melt butter over medium-high heat and swirl to coat the bottom of the pot. Add leeks and sliced shiitake and cook, stirring, until softened, about 2 to 3 minutes.
Add ginger, garlic, mirin, sesame oil and soy sauce and cook for 1 minute.
Turn heat to high, add reserved broth and bring to a simmer.
Add bok choy and reserved noodles and cook for 1 to 2 minutes, until greens are cooked and noodles are heated through.
Gently stir in miso and tofu. Turn off heat.
Using tongs, divide noodles among 4 to 6 bowls. Ladle hot soup over noodles. Sprinkle with scallions and serve.
Pass togarashi or crushed red pepper at the table.