Preheat your oven to 350 degrees Fahrenheit. Spray a nine inch quiche dish with cooking spray and set aside.
In a large skillet over medium heat, heat the olive oil. Then add the garlic, onion, and mushrooms. Cook, sautéing until the mushrooms are soft. About 5 minutes.
1 tablespoon extra virgin olive oil, 2 garlic cloves, 1/2 yellow onion, 8 ounces cremini mushrooms
Add in the spinach and stir until wilted. About two minutes. Remove from the heat and set aside.
5 cups spinach
In a large bowl whisk together the eggs, egg whites, milk, salt, pepper, and cheese.
4 large eggs, 4 egg whites, 3/4 cup swiss cheese, 3/4 cup milk, 1 teaspoon salt, 1/2 teaspoon black pepper
Pour the vegetables into the quiche dish. Add the egg mixture over the top. Bake for 40 minutes or until the edges start to pull away from the side of the dish and the middle has just a slight jiggle to it.