Roll out the pie crust dough to about a 12-inch diameter circle. Place the dough into a 9-inch pie plate, gently forming it to fit the plate. Then, cut off the extra dough hanging over the edge of the plate.
Line the pie crust with parchment paper (see note) and fill with pie weights of dried beans. Bake for 8 minutes. Remove from the oven and set aside.
Place a large skillet over medium heat and add the olive oil. Once hot, add the diced onion and cook for 2-3 minutes.
Add the mushrooms and cook for another 2-3 minutes. Add the spinach and cook until it's wilted. (You may need to add the spinach in batches as it wilts depending on the size of your skillet.) Drain any liquids from the pan while pressing on the veggies gently to help remove any excess moisture.
Stir in 1/2 teaspoon salt, 1/2 teaspoon pepper, and 1/4 teaspoon nutmeg. Remove from the heat and set aside.
Whisk the eggs, cottage cheese, and milk together in a medium mixing bowl. Stir in the cooked veggies, shredded Swiss cheese, 1 teaspoon salt, 1/2 teaspoon pepper, and 1/4 teaspoon nutmeg.
Pour the quiche filling into the par-baked pie crust.
Bake for 45-60 minutes or until the quiche is set and the crust is golden brown.
Remove from the oven and rest for 10 minutes on a wire rack before cutting into slices and serving.
Notes
Before placing the parchment paper over the pie dough, crumple it up and then spread it back out. This will make it easier to form the paper to the pie plate shape and keep it from cutting into the dough. Pie weight/dried beans are used to help keep the sides from falling down while blind baking.