In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 3-4 minutes.
Stir in the minced garlic and ginger, cooking for an additional 1-2 minutes until fragrant.
Add the smashed lemongrass, coconut milk, vegetable broth, fish sauce, lime juice, and red pepper flakes to the pot. Bring the mixture to a gentle simmer.
Add the clams to the pot, cover, and cook for about 5-7 minutes, or until the clams have opened. Discard any clams that do not open.
Remove the pot from heat and discard the lemongrass stalk. Ladle the broth and clams into bowls and garnish with fresh cilantro.
Serve with lime wedges on the side for an extra burst of flavor.