Cook the noodles in a large pot of boiling salted water until tender yet chewy, 3 to 5 minutes. Drain and rinse under cold water to stop the cooking. Divide the noodles among 4 wide bowls.
Meanwhile, heat the broth in a medium saucepan over medium-high heat until barely simmering. Add the mushrooms and cook until tender, about 5 minutes. Add the snap peas and soy sauce, and cook 1 minute.
Whisk together the miso and ¼ cup of the soup broth in a cup until dissolved. Remove the soup from the heat and stir in the miso mixture and scallions. Pour over the noodles in each bowl and top with the eggs, cilantro, and chili oil.
Tip: You can replace the thin somen noodles with thicker udon noodles.