Make the dressing by stirring all the dressing ingredients together in a bowl; set aside.
Wash the clams in cold water and discard any that are broken or will not close their shells when tapped. If you are using wild clams, there may be sand inside the shells, but farm-raised clams usually don’t have this problem. To get the clams to spit out sand, feel free to use your favourite method, but I like to soak them in cold water with salt for 30 minutes to an hour. After the clams have released the sand, rinse them again in cold water.
In a pot just big enough to hold the clams, add 1 inch of water and bring to a boil. Add the lemongrass and clams, then cover and cook for 1–2 minutes until the clams open. Remove from the heat and transfer the clams into a large mixing bowl, leaving the cooking water in the pot (it’s fine if some of the lemongrass comes along with the clams). If some of the clams did not open, return them to the pot, cover, and cook them for another minute or so—some clams are just late bloomers! Discard any that still won’t open after the second cooking.
Spoon out 3 Tbsp of the clam cooking liquid and add to the dressing; stir to mix and set aside.
Return the pot to the stove, bring the remaining cooking liquid to a boil, and blanch the seafood of your choice in this liquid, adding more water if needed. Add the cooked seafood to the clam bowl. Repeat with the remaining seafood (see Tip for Success below).
Toss the Thai basil and shallots with the seafood while it’s still warm and let the heat wilt the shallots and infuse the basil’s aroma. Pour the dressing over the seafood, then add the grape tomatoes, green onions, and cilantro; toss gently.
Serve in a shallow bowl on its own as an appetizer, or with jasmine rice.