Step 1

In a large saucepan, combine the shrimp shells with the onion, carrot and 8 cups of water and bring to a boil. Simmer over moderate heat until the stock is bright orange and reduced to 5 cups, about 30 minutes. Strain and reserve the stock.

Step 2

In a food processor, combine the shallots, jalapeños, ginger, macadamia nuts, coriander and turmeric with 2 tablespoons of the fish sauce and 2 tablespoons of the canola oil; puree until smooth.

Step 3

In a soup pot, heat the remaining 2 tablespoons of oil. Add the seasoning paste and cook over moderate heat, stirring, until fragrant, about 2 minutes. Add the lemongrass and cook, stirring occasionally, until the mixture darkens slightly and the oil separates, about 10 minutes.

Step 4

Add the reserved shrimp stock, coconut milk, brown sugar and the remaining 2 tablespoons of fish sauce. Simmer over moderate heat until the soup is reduced to 6 cups, about 15 minutes.

Step 5

Meanwhile, in a large pot of boiling water, cook the rice noodles just until pliable, about 1 minute. Drain, shaking out the excess water. Transfer to 8 soup bowls.

Step 6

Add the shrimp to the soup and cook until pink and curled, about 5 minutes. Season the soup with salt and ladle it over the noodles. Garnish with lime wedges and serve.