Preheat your oven to 375°F (190°C) and lightly grease a 9-inch pie dish with a small amount of oil.
Heat the olive oil in a skillet over medium heat, then add the finely diced yellow onion and minced garlic, sautéing until they are soft and translucent.
Add the fresh spinach to the skillet and cook until it is completely wilted, then transfer the mixture to a colander and press firmly with a spoon to remove all excess moisture.
In a large mixing bowl, whisk together the whole eggs, liquid egg whites, and non-fat Greek yogurt until the custard base is smooth and well-combined.
Stir the sea salt, black pepper, and ground nutmeg into the egg mixture until the seasonings are evenly distributed.
Spread the prepared spinach and onion mixture across the bottom of the greased pie dish and sprinkle the shredded Swiss cheese evenly over the top.
Pour the egg and yogurt mixture over the vegetables and cheese, using a fork to ensure the filling is submerged.
Bake the quiche for 30 to 35 minutes, or until the center is set and the top has turned a light golden brown.
Remove from the oven and let the quiche rest for at least 5 minutes before slicing to allow the texture to firm up.