Preheat oven to 375 degrees, and spray a 9x13 baking dish with non-stick cooking spray.
Heat the oil in a large non-stick frying pan. Add all the spinach and stir until just wilted, about 2 minutes.
1 tsp olive oil, 4 cups packed spinach
Transfer the spinach to the casserole dish, spreading out so that the bottom is covered.
Sprinkle the onions evenly out over the spinach, followed by the shredded cheese.
1/3 cup green onions, 1 1/2 cups shredded cheddar cheese
Salt and pepper the eggs, to taste.
salt & pepper, 12 eggs
Pour the eggs out over the spinach, cheese, and onions.
Using a butter knife, give it a quick swirl around the pan to help the eggs settle and ingredients combine.
Bake about 35 minutes or until the mixture is completely set and starting to slightly brown.
Let cool about 5 minutes before cutting and serving to allow everything to settle.