Preheat the oven to 350 degrees. Coat a tart pan with cooking spray.
Heat the olive oil in a large skillet over medium high heat. Add the onions and mushrooms then cook stirring often until caramelized, about 15-20 minutes. Add the spinach to the skillet then season with sea salt and freshly cracked pepper, to taste. Toss until well combined.
Place some of the mushroom & onion mixture and Swiss cheese into the bottom of the tart pan. Combine the eggs with the milk then season with sea salt and freshly cracked pepper, to taste. Whisk the eggs until they are mixed thoroughly.
Pour the egg mixture into the tart pan then add the remaining onion mixture and Swiss cheese to the top.
Place into the oven and bake for 20-25 minutes, or until a tester inserted in the center comes out clean. Let the frittata cool for a few minutes before slicing and serving. Enjoy.