Spray a 12-cup muffin tin generously with cooking spray.
In a small skillet over medium heat, warm the oil. Add the onion and cook, stirring frequently, until softened, about 7 minutes. Add the spinach and cook until wilted, about 1 minute longer.
Transfer to a cutting board to cool, then chop the mixture so the spinach if broken up a little.
When ready to cook, set the temperature to 350°F and preheat, lid closed for 15 minutes.
In a large bowl, whisk the eggs until frothy. Add the cooled onions and spinach, cheese, basil, 1 tsp salt and 1/2 tsp pepper. Stir to combine.
Divide egg mixture evenly among the muffin cups.
Place tray on the grill and bake until the eggs have puffed up, are set, and are beginning to brown, about 18 to 20 minutes.
Serve immediately, or allow to cool on a wire rack, then refrigerate in an air tight container for up to 4 days. Enjoy!