Heat the oil in a large, deep skillet until shimmering. Add the onion and cook over moderate heat, stirring occasionally, until softened and just beginning to brown, about 7 minutes. Add the ginger, garlic and curry and cook, stirring, until fragrant, about 2 minutes. Add the coconut milk and stock and simmer over moderate heat until reduced by one-third and thickened, about 8 minutes. Add the shrimp, season with salt and cayenne and simmer over moderate heat until the shrimp are pink and cooked through, about 5 minutes. Stir in the mint and lemon zest and serve in deep bowls over steamed rice.