Greek Egg Casserole

Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch casserole dish with cooking spray.

Melt butter in a medium skillet over medium heat. Add onions and cook, stirring occasionally, until soft, 5 to 7 minutes.

Meanwhile, heat another pan over medium heat. Cook spinach until heated through, about 5 minutes. Drain.

Whisk eggs, milk, salt, and pepper together in a large bowl until well combined and foamy. Add the onions, spinach, tomatoes, and feta cheese and stir to combine. Pour egg mixture into the prepared dish.

Bake in the preheated oven until eggs are set, 30 to 35 minutes.