How to Make It
In a large saucepan of boiling salted water, cook the udon until al dente, about 6 minutes. Drain, shaking out the excess water.
Meanwhile, in a large saucepan, combine the garlic, sesame oil, 1 1/4 teaspoons chili sauce, chile powder, sugar and a pinch of salt. Add the clam broth and water and bring to a boil. Add the clams and cook until opened, about 5 minutes; transfer the clams to a bowl.
Add the spinach, scallions and udon to the broth and cook until the spinach is wilted, 1 minute. Ladle the udon soup into 4 bowls and top with the clams. Pass extra chili sauce at the table.