Add the coriander seeds, cardamom, turmeric, cinnamon, chile flakes, lemongrass, and ginger. Heat through, stirring, until aromatic, then add the stock and coconut milk and bring to a gentle simmer. Taste and adjust the seasoning as necessary. Simmer for 8–12 minutes until the sauce is flavorful and thickened and the meatballs are cooked through.