In large pot set over medium-high heat, cook onions in oil until tender, about 2-3 minutes. Add curry paste and cook, stirring, 1-2 minutes more. Add coconut milk, lime juice and sugar. Simmer 3-4 minutes, and then add mussels. Cover, and cook until they just open. Divide mussels and cooking liquid among 4 bowls. Sprinkle with cilantro or green onions and garnish with lime wedges. Mussels could also be served on rice or Asian-style noodles.
Note: Curry paste, which comes in varying degrees of hotness, can be found in the Asian food aisle. Discard any mussels that do not tightly close when squeezed or tapped before cooking. Discard any that do not open during cooking. Both situations indicate the shellfish is dead and should not be eaten.