Make sure to take the puff pastry and spinach out ahead of time to thaw (spinach can be thawed quickly in the microwave, if needed, but do not thaw the puff pastry in the microwave). Preheat the oven to 375ºF.
Dice the onion and mince the garlic. Add the onion, garlic, and olive oil to a skillet and sauté over medium heat until the onions are soft and translucent. Set the onions and garlic aside to cool slightly.
Add the cottage cheese, Parmesan, nutmeg, salt, pepper, and two of the eggs to a bowl, and stir to combine (save the third egg to brush on top of the pastry before baking).
Squeeze out as much water as possible from the thawed spinach. Add the squeezed spinach and sautéed onion and garlic to the bowl with the cheese mixture. Stir to combine again.
Place a sheet of parchment paper on your work surface (or dust lightly with flour) and roll the thawed puff pastry out until it’s just slightly larger, about 10x10 inches.
Drape the rolled pastry dough out over a pie plate. Spread the spinach and cheese mixture inside the pastry dough, then take the corners and fold them in toward the center. The pastry will not fully cover the top, but that’s okay (see step by step photos below).
Lightly whisk the remaining egg and brush it over the surface of the pastry dough. Bake the spinach pie for about 45 minutes, or until the pasty on top has puffed up significantly and turned golden brown.
Remove the spinach pie from the oven, let sit for about five minutes, then slice into eight pieces and serve.