Peel and dice potato and simmer in boiling salted water for 10 minutes until tender. Meanwhile cut a slice off the top of your tomatoes. Scoop out the insides and turn upside down to drain.
Drain and mash potato with well-drained and defrosted frozen spinach, butter, a nice pinch of grated nutmeg and seasoning. Finely chop a St. Nectaire and fold into spinach mixture with 1 egg yolk. Whisk 1 egg white and fold into mixture. Spoon into tomato shells set in a small roasting tin. Bake at 375°F, 12-15 minutes until well risen and golden. Serve immediatly.