Heat over to 425 degress. Fit the pie crust into and up the sides of a pie plate; fold the edge of dough underneath itselt to create a thicker 1/2-in. border that rests on the lip of the plate and crimp as desired. Place on a rimmed baking sheet and bake until lightly golder, 12 to 15 minutes. Remove and reduce oven heat to 375.

Meanwhile, heat the oil in a large skillet over medium heat. Add the onion and 1/4 tsp each salt and pepper and cook, stirring occasionally, until soft, 5-7 minutes. Stir in parsely and remove from heat.

In a large bowl, whisk together the eggs, sour cream, milk, 1/4 tsp each salt and pepper, and nutmeg, if using. Gently stir in the onion mixture and Gruyere/Swiss.

Pour the egg mixture into the crust. Bake until just set and a knife inserted in the center comes out clean, 35-40 minutes. Let cool 5 minutes before serving.