Preheat oven to 350 degrees F (175 degrees C). Spray muffin cups with cooking spray.
Heat olive oil in a skillet over medium heat; cook and stir spinach until wilted. Remove from heat and cool spinach. Squeeze spinach to remove excess moisture.
Whisk egg whites and egg yolks together in a large bowl; add Parmesan cheese, Mexican cheese blend, garlic powder, sea salt, and spinach and mix well. Pour egg mixture into the muffin cups almost to the top. Place the muffin tin on a rimmed baking sheet and pour water halfway up the sides of the muffin tin to create a water bath.
Bake in the preheated oven until muffins are set in the middle, 20 to 25 minutes.
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