Preparation
In a large saucepan, bring 4 quarts of salted water to a boil.
Add the chayotes, cover, and cook until they are tender when pierced with a skewer, about 30 minutes.
While the chayotes are cooking, prepare the sauce.
In a small bowl, whisk together the flour and skim milk until smooth.
In a large non-aluminum saucepan, stir together the remaining skim milk and the evaporated skim milk. Bring to a simmer over medium heat.
Very gradually whisk in the flour mixture. Stir until the sauce boils and thickens.
Reduce the heat to very low and cook for 2 minutes, stirring occasionally.
Stir in the sour cream, parsley, and scallions and season to taste with salt (optional) and plenty of pepper.
Drain the chayotes and cut into ¼-inch thick slices.
Put the sliced chayotes in a serving bowl, pour the sauce over, and serve immediately.
