Preheat oven to 350 degrees. Spray a 9" pie pan with nonstick spray and set aside.
Bring a large skillet sprayed with nonstick spray to medium-high heat on the stove. Add onion and, stirring occasionally, cook until softened and slightly browned, about 6 minutes.
Transfer cooked onion to a large bowl and set aside. Remove skillet from heat. If needed, wash and dry. Spray with nonstick spray and return to medium-high heat. Stir occasionally, cook until softened and lightly browned, about 4 minutes. Add spinach and cook until wilted and excess moisture has evaporated, about 2 more minutes.
Transfer spinach mixture to the large bowl. Add cheese wedges, breaking them into pieces as you add them. Mix thoroughly, until cheese has melted and is evenly distributed. Add all remaining ingredients to the bowl and mix well. Evenly transfer mixture to the pie pan.
Bake in the oven until firm and lightly browned on top, about 1 hour.
Allow to slightly cool. Cut into 8 slices and enjoy!