Reduce the heat to medium and add the remaining 2 tablespoons of the oil to the skillet. Add the squash and cook, stirring, for 1 minute. Add the water, cover and cook until the squash is nearly tender, about 4 minutes. Remove the lid, add the onion and cook, stirring, uncovered, until softened, about 2 minutes. Add the apple slices and cook, stirring, for 1 minute more.
