Natural cooking spray
1 Tablespoon extra-virgin olive oil*, divided
2 cups (about 1 onion) thinly sliced yellow onion
1 pound button mushrooms, thinly sliced
3/4 teaspoon salt and 1/2 teaspoon freshly ground black pepper, divided
6 large eggs
3/4 cup reduced-fat milk
1 teaspoon freshly grated nutmeg
1 1/2 cups shredded Swiss cheese*
1 (16-ounce) bag frozen chopped spinach*, thawed and squeezed to remove as much liquid as possible
Preheat oven to 350°F and grease a 9-inch square pan with cooking spray; set aside.
Heat olive oil in a large skillet over medium-high heat.
Add onion and cook until golden brown, 7 to 8 minutes.
Add mushrooms, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook until liquid is absorbed, 10 to 12 minutes; set aside to cool.
In a large bowl, whisk together eggs, milk, nutmeg and remaining 1/4 teaspoon salt and 1/4 teaspoon pepper.
Add cheese, onion, mushrooms and spinach and mix well.
Transfer to prepared pan, spread out evenly and bake until set in the middle and golden brown, about 45 minutes.
Let cool 10 or 15 minutes, then cut into 24 squares and serve.