This appetizer is delicious with musselsaswell. Decrease the amount of chiles if you like it less spicy.
Dissolve 2 tablespoons salt in a large bowl of cold water. Add clams. Let soak at room temperature 15 minutes. Drain.
Scrub clams under cold running water. Transfer to a large bowl of fresh cold water. Repeat, scrubbing clams and rinsing in fresh water. Drain. Refrigerate, uncovered, until ready to use, up to 8 hours.
Put reserved lemongrass leaves in a large pot with 1/2 inch of water. Bring to a boil. Add clams. Steam, covered, until clams open, 6 to 7 minutes. Drain. Discard any unopened clams and the leaves.
Add oil, lemongrass, shallots, garlic, and chiles. Cook over medium-high heat, stirring constantly, until lemongrass and shallots turn golden, about 2 minutes. Remove from heat. Add clams to pot, and gently stir to coat.
Stir together lime juice and fish sauce in a small bowl; pour over clams. Transfer clams to a large serving bowl; sprinkle with cilantro. Garnish with lime wedges.