1. Chop the onion, ginger, garlic, shrimp shells, and lemongrass finely by hand or add to a blender or food processor. Purée to make a paste, scraping down the sides as needed.
2. Heat a heavy soup pot or Dutch oven over medium heat for a couple minutes until hot. Add the oil and then the paste mixture along with the coriander and cumin and sauté, stirring often, until it softens, becomes very aromatic, and deepens in color, about 5 minutes. 
3. Add the chicken broth, sugar, and fish sauce and bring to a boil. Lower the heat to medium low, and cover with a lid. Simmer gently for 15 minutes. Strain the broth through a fine sieve, pressing on the solids to extract as much liquid as possible. Discard the solids.
4. Rinse the pot and return the broth to the pot. Add the coconut milk, season with salt to taste, and return to a simmer. (The soup should be somewhat salty. Once you add the noodles and veggie garnishes, it will taste balanced).

Arrange the garnish plate:
1. Place the bean sprouts, sliced chiles, limes, and herbs onto a plate and set aside.

Make the noodles:

1. Bring a large pot of water to a boil. Add the rice noodles and cook for about 2 mins. until cooked through but still firm (but follow package instructions to be sure. There are many different thicknesses of rice noodles). Drain, rinse, and distribute among two large soup bowls.
2. Just before serving, add the shrimp to the broth and gently simmer until they’re just cooked through, about 1 minute. Ladle the hot soup and shrimp over the noodles.
3. You can either garnish the bowls with the herbs, or let people do it themselves at the table. .
4. Serve lemongrass shrimp noodle soup immediately.