Soak the noodles in warm water according to the package directions until soft. Meanwhile, heat up the stock and add the chiles, garlic, ginger, lemon zest and juice and fish sauce and bring to a simmer. Add the noodles and mushrooms and cook for a few minutes, until the noodles are tender. Remove them from the stock with tongs, divvying them up between 4 shallow bowls.
Add turkey or chicken and coconut milk and bring to a simmer (if you're using raw chicken, simmer for 3 minutes, until the chicken is cooked through. Season with salt. Add the spinach and cook just until it wilts. If you like, fish out the slices of chili and/or lemongrass. Stir in some of the cilantro, saving some for the top.
Ladle the soup over the noodles and serve hot.