Meanwhile, heat the oil in a medium skillet. Add the shallot and cook over moderate heat until softened, about 2 minutes. Add the peanut butter, red curry paste and the remaining 1 1/4 cups of coconut milk and 1 teaspoon of fish sauce. Simmer over low heat until slightly thickened, about 2 minutes. If the sauce breaks, whisk in the boiling water. Add the lime juice and season with salt.