Coarsely chop the ginger and garlic in a food processor. Add 1/2 cup (125 mL) water and process until almost smooth. Set aside.

Heat the oil in a large heavy saucepan that's deep enough to hold all the shellfish. Add the onion, cover and cook over low heat, stirring occasionally, until softened, about 10 minutes. Add the jalapeno, lemon grass, cumin, coriander, turmeric and the garlic and ginger puree. Cook mixture over medium heat, stirring, about 3 minutes, or until most of the liquid has evaporated. Stir in coconut milk, the remaining water and salt and pepper to taste. Bring to a simmer. Sauce can be prepared ahead of time and refrigerated until ready to cook shellfish.

Stir in the mussels and clams. Cover and cook over medium heat, just until the mussels and clams open, about 5 to 7 minutes. Discard any shellfish that didn't open. Stir in the lime juice. Spoon mussels, clams and broth into shallow bowl. Serve immediately with lime wedges.