Preheat oven to 350 degrees F. Grease a 12 standard muffin cups with nonstick cooking spray. Set aside. 

Heat olive oil in a large nonstick skillet over medium-high heat until it shimmers. Add the spinach. Cook, stirring frequently, until soft and wilted. Remove pan from heat and allow spinach to cool, about 5 minutes. Place spinach on a cutting board and roughly chop. 

Whisk eggs in a large mixing bowl. Stir in chopped spinach, salsa, salt, and pepper. Divide evenly among greased baking cups. 

Bake until set, about 18 minutes. Remove from oven. Allow egg cups to cool in the pan for five minutes. Run a sharp knife around the edge of each cup to loosen. 

Store leftovers wrapped in the refrigerator for up to three days. Serve chilled or reheat gently in the microwave on low power.