Method

1. Bring a large pot of water to boil and salt it. Put the kombu and dried shiitakes in a bowl and cover with 2 cups boiling water from the pot. 

2. Trim any tough stems from the fresh mushrooms and reserve them; slice the caps and tender stems. In another pot, combine 6 cups water, the mushroom stems, carrots, onion, ginger, bay leaves, soy sauce, and five-spice powder and bring to a boil. Adjust the heat so the mixture bubbles steadily and let it simmer while you prepare the remaining ingredients, at least 20 minutes or up to 1 hour. 

3. Put the oil in a large skillet over medium-high heat. When it's hot, add the sliced mushrooms and sprinkle with salt and pepper. Cook, stirring frequently, until the mushrooms release their liquid and the pan is dry, 3 to 5 minutes. Lower the heat to medium and cook, stirring once in a while, until the mushrooms shrink and crisp, another 10 to 15 minutes. Turn the heat to low.

4. While the mushrooms crisp, add the noodles to the boiling water and cook, stirring occasionally, for 5 minutes, then start tasting. The noodles are done when they are just tender but a little firmer than you ultimately want them (watch brown rice noodles closely; they go from firm to mushy in a flash). Drain the noodles in a colander, rinse them under cold tap water until cooked, and then put them in a bowl covered with cold water until you're ready to assemble the soup. 

5. Strain the kombu and mushroom-soaking liquid into the large pot used to cook the noodles. Discard the kombu. Chop the rehydrated shiitake mushrooms, add to the skillet with the sauteed mushrooms and stir.

6. Strain the stock into the pot with the mushroom-soaking liquid and discard the solids; turn the heat to medium-high. Add a splash of stock to the skillet and scrape up any browned bits from the bottom of the pan, then add the mushroom mixture to the stock along with the lime juice. Bring the mixture to a boil, then turn off the heat. Taste and adjust the seasoning. 

7. Drain the noodles, run them under hot tap water, and drain them again. Divide the noodles, scallions, and watercress evenly among 4 bowls and ladle the broth over. Serve right away, passing the chile at the table for garnish.