4 quarts water
1/2 lb. fresh or 3/4 lb. dried udon noodles
1 lb. pork tenderloin, partially frozen
2 quarts low-sodium beef or chicken broth
1 carrot, peeled and cut into julienne
4 fresh shiitake mushrooms, brushed clean, stems removed and caps thinly sliced
1 1/2 oz. enoki mushrooms, trimmed and rinsed
1 Tbs. peeled and grated fresh ginger
1 or 2 fresh red chilies
1/4 cup mirin
1 Tbs. soy sauce
1 tsp. sugar, or to taste
1 bunch watercress, stems removed
1 bunch green onions, including 2 inches of green tops, thinly sliced
1/4 cup sesame seeds, lightly toasted
In a large pot over high heat, bring the water to a rapid boil. If using fresh udon noodles, add them to the boiling water and cover the pot. When the water returns to a boil, uncover and cook for about 10 seconds. Drain the noodles in a colander, rinse under cold running water and set aside. If using dried udon noodles, add them to the boiling water, stir well and cook, stirring occasionally, until just tender, about 5 minutes. Drain the noodles in a colander, rinse under cold running water and drain again thoroughly. Set aside.
Using a sharp knife, cut the pork against the grain into thin slices, then cut the slices into matchsticks. In a large saucepan over high heat, combine the broth, carrot, shiitake and enoki mushrooms, ginger, chilies, mirin, soy sauce and sugar and bring to a boil. Reduce the heat to medium and simmer for 2 minutes. Add the pork, increase the heat to high and cook for 1 minute. Stir in the cooked noodles, watercress and green onions, and simmer until the watercress and green onions have wilted and the noodles are just heated through, about 1 minute.
Remove the pot from the heat, and remove and discard the chilies. Taste and adjust the seasonings, then ladle the soup into warmed bowls. Sprinkle with the sesame seeds, dividing evenly, and serve immediately. Serves 4.