Coconut cream is what gives this soup its velvety richness. Otherwise it’d just be green juice and noodles! Add the fresh herb purée to any clear broth to give it a shock of green.
2 serrano chiles, 1 coarsely chopped, 1 thinly sliced
1 3-inch piece ginger, peeled, thinly sliced
2 lemongrass stalks, bottom third only, tough outer layers removed, thinly sliced
2 garlic cloves, smashed
1 tablespoon fish sauce
1 tablespoon light brown sugar
1 5.4-ounce can coconut cream
3 cups cilantro leaves with tender stems
3 cups basil leaves, plus more for serving
2 tablespoons virgin coconut oil or vegetable oil
36 littleneck clams (about 4 pounds), scrubbed
8 ounces rice stick noodles
Purée chopped chile, ginger, lemongrass, garlic, fish sauce, brown sugar, and 3 cups water in a blender until smooth. Transfer curry to a small bowl. Reserve blender (no need to clean).
Purée coconut cream, cilantro, 3 cups basil, and ¼ cup ice water in blender until very smooth; set herb purée aside.
Heat oil in a large saucepan or small pot over medium-high. Add clams, cover, and cook until clams open, 5–7 minutes. Uncover and transfer opened clams to a medium bowl, reserving cooking liquid in saucepan. If any clams are still closed, cover and cook a few minutes longer, then add to bowl with others; discard any clams that refuse to open.
Add curry base to saucepan and bring to a boil. Reduce heat and simmer 10 minutes to let flavors meld.
Meanwhile, cook noodles according to package directions. Drain and rinse under cold water. Divide among bowls.
Using a slotted spoon, transfer clams to noodle bowls. Pour any accumulated clam-cooking liquid into curry mixture and stir in herb purée; bring to a boil. Immediately remove from heat and squeeze in juice from lime. Taste and season with salt if needed. Ladle curry over clams and noodles. Top with sliced chile and basil leaves.