Preheat oven to 375 degrees, place 9 liners in a muffin tin and spray the liners generously with cooking spray. Set aside.
Rinse baby spinach in a large strainer and leave wet. Heat a large skillet over medium-high heat. Add the wet spinach and cover. Let steam 2-3 minutes, stirring occasionally. When wilted and bright green, pour spinach back into strainer and press with a wooden spoon to extract as much water as possible. When cool enough to handle, chop finely.
In a large measuring cup or a bowl with a pour spout, beat eggs and milk until color is uniform then stir in salt and pepper.
Place an equal amount of tomatoes, spinach and feta cheese crumbles into each muffin cup then fill cups approximately 80% full with beaten egg.
Bake 14–16 minutes or until centers are barely firm to the touch and the tops of the muffins have domed. Remove from oven and allow them to cool 2–3 minutes before serving. Enjoy!