Preheat oven to 375°. Very lightly butter a 9″x13″ microwave-safe baking dish (using part of the 6 tbsp. called for in the recipe is perfectly acceptable!).
Roughly chop the spinach and transfer to the prepared dish. Cube the butter and distribute over the spinach in the dish.
Cover the dish tightly with plastic wrap and microwave for 3–4 minutes to wilt the spinach. After steaming, the spinach should be halved in volume and still bright green.
Meanwhile, whisk the eggs in a very large bowl until the yolks and whites are well broken up.
Add all the remaining ingredients and whisk until evenly combined.
Remove the plastic wrap and pour the egg and cheese mixture over the wilted greens.
Use a fork or knife to swirl the greens through the eggs.
Bake for 45–60 minutes, until the center is set and the top is puffed and golden. The center should not jiggle when the pan is shaken.
Allow to cool for 10 minutes before cutting and serving.