Place grated zucchini in colander, toss with 1 t salt, and set in sink or over a bowl to drain.
Heat oil, reduce heat to medium-low, add potatoes, onion, and ½ t salt, and sauté, stirring occasionally, for 30 minutes. If potatoes start to brown, reduce heat.
Squeeze handfuls of zucchini to get rid of as much water as possible, and add the zucchini to the potatoes. Sauté, stirring occasionally, for another 15 minutes. Remove from heat, and use the slotted spoon to transfer vegetables to a large bowl, allowing excess oil to remain in the pan.
In the small bowl, beat the eggs with remaining ½ t salt, nutmeg, and pepper, then stir into the vegetables.
Pour into pan, press everything down lightly, and cook, covered, over very low heat. After 7 minutes, use a rubber spatula to gently lift up one side and check the color of the bottom. When the tortilla is just about set and the bottom is lightly golden (no more than 12 minutes total), remove from heat and let stand, still covered, 5-10 minutes, until set.
Run rubber spatula all the way around the pan to make sure the tortilla isn’t stuck anywhere. (This should be easy if you use a non-stick pan.) Place plate upside-down over pan and quickly (and carefully!) flip both, so that the top of the tortilla is now the bottom.
Slide tortilla back into the pan and cook over low heat for another five minutes. Let cool and serve.