While the meatballs are cooking, make the curry sauce. Heat the oil on medium-high in a deep, wide skillet or dutch oven. Add the onion and cook, stirrinq often, until the onion turns soft and translucent, about 5 to 7 minutes. Stir in the garlic and ginger and cook until fragrant, about 1 minute. Add the red curry paste and cook, stirring everything around, until the paste sizzles and turns a darker red, about 3 minutes. Stir in the coconut milk, chicken stock, fish sauce, sugar and peanut butter and bring sauce to a boil, then lower the heat and gently simmer until the sauce thickens slightly, about 5 minutes. Add in the spinach and lime juice and stir until wilted, 1 minute. Transfer the meatballs to the sauce and gently toss until coated in sauce.