Heat oven to 375 degrees.
In a large skillet, over medium-low heat, heat the oil.
Add the onions, 1/2 tsp of the salt, and 1/4 tbs of the pepper.
Cover and cook until the onions are softened, 5 to 7 minutes.
Add the parsley and cook, covered, for 2 minutes more.
Meanwhile, whisk together the eggs and half-and-half.
Stir in the Gruyere, nutmeg, the remaining salt and pepper, and the onion mixture.
Place the piecrust on a foil-lined baking sheet.
Scrape the egg mixture into the piecrust; it will be very full.
Bake until the filling is set and a knife inserted into the center comes out clean, about 40 minutes.
Let rest for 5 minutes.
Cut into wedges and serve.