Mushroom Udon Noodle Bowl

Name: Mushroom Udon Noodle Bowl
Ingredients: 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 (1-inch) chunk of ginger, smashed and unpeeled; 2 whole, unpeeled garlic cloves; Stems or trimmings from fresh shiitake mushrooms (see below) (optional); 1 teaspoon salt; 1 pound dried udon noodles (or use fresh or frozen, cooked udon); 2 tablespoons butter; 1 cup chopped leeks, white and pale green parts only; 8 ounces fresh shiitake mushrooms, stems removed, caps sliced in 1/4-inch 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