Heat the oil in a large enameled cast-iron casserole. Add the onions, cover and cook over low heat, stirring occasionally, until softened, about 10 minutes. Add the jalapeños, lemongrass, cumin, coriander, turmeric and the garlic and ginger puree and cook, stirring, for 2 minutes. Stir in the evaporated milk and coconut milk and bring to a simmer. Remove from the heat and let stand for at least 30 minutes or for up to 4 hours.