Bring the parsnips to a boil in a pan of salted water, then simmer for 10 to 15 minutes. You want them to be tender, but not mushy at all.
Drain the parsnips and allow them to cool until they are handleable. Cut them diagonally into 2" long x 1/2" or 1/4" thick slices.
Place the flour in a large mixing bowl and make a well in the center. Place the egg yolk into the well and mix it in with a fork, gradually pulling in the flour from the edges of the bowl. Begin adding the milk slowly, mixing all the while. Add salt, pepper and cayenne to taste. Beat mixture with a whisk until it's a smooth batter.
Wash and dry the spinach leaves, and place them in a bowl.
Combine the olive and walnut (if using) oils, sherry vinegar, salt and pepper (to taste) in a dressing cruet with a secure top. Set aside.
Just before serving time, whisk the egg white to soft peak stage. Fold a little bit of the yolk batter into the whites, then fold the egg white mixture into the yolk batter.
Heat an inch or two of cooking oil in a deep-sided heavy skillet until is shimmers but isn't smoking (or set a deep fryer to manufacturer's recommended temp) .
While oil is heating, shake the dressing vigorously to combine, then toss the spinach and dressing together. Arrange the dressed spinach on 4 salad plates, then scatter the chopped walnuts over them.
Once oil is hot, dip the sliced parsnips into the batter a few at a time, then fry them until puffy and golden. Drain them in a paper towel lined bowl and keep them warm while you fry the rest of the parsnips. Arrange the hot parsnips over the dressed spinach and serve immediately.