Heat the oil in a large skillet and add the chopped onion.
Cook over medium heat 5 minutes or until the onions are soft and starting to carmelize.
Add the spinach and cook until warm and moisture has evaporated.
In a medium size bowl, whisk together the eggs, cheese, heavy cream, mustard powder, salt, and pepper.
Stir in the spinach and onions.
Grease a pie pan with butter or coconut oil and add the spinach egg mixture.
Bake 35 minutes until the center is set.
Notes
You can use fresh spinach in place of frozen, but keep in mind that because it cooks down so much, you’ll need a substantial amount. If you’d like to use an oven-proof skillet to cook the spinach and onions, pour in the egg mixture, and put the whole thing in the oven without dirtying a pie dish, reduce the cook time by about 10 minutes.