Put vinegar in a large bowl with the feta. Use a fork to mash the cheese a little and stir them together. Add the nutmeg and the black pepper and stir again. Keep stirring while you slowly pour in the olive oil.
Chip the spinach leaves into small pieces, add to the bowl and toss to coat with the dressing. Taste and adjust the seasoning, adding salt if necessary.
Divide among salad bowls, top each with a dollop of yogurt and a dusting of nutmeg, and serve.