Heat the oil in a medium saucepan over medium heat. Add the onion and saute until translucent, 7 to 8 minutes. Stir in the ginger and garlic and cook 1 minute. Stir in the red curry paste, then add the coconut milk and fish sauce. Whisk to combine. Bring the mixture to a low boil, cover, reduce the heat and simmer for 10 minutes. Add the meatballs and simmer an additional 10 minutes.
Turn off the heat, stir in the lime juice and transfer to a serving bowl. Garnish with the cilantro and scallions.
Vegetarian fish sauce and curry paste is available. You can also substitute soy sauce for the fish sauce.