Preheat your oven to 400°F. Place an ungreased ceramic or glass 9-inch pie plate in the preheating oven, allowing it to preheat with the oven.
In a large bowl, whisk the eggs with kosher salt, black pepper, garlic powder, dried thyme, and red pepper flakes.
Stir in the parmesan and the chopped onion. Then stir in the spinach in batches.
Using oven mitts, pull the hot pie plate out of the oven and place it on a trivet. Lightly grease the pan.
Pour the egg and spinach mixture into the hot pie plate. Use a rubber spatula to smooth out the top. It will seem like a lot of spinach relative to the eggs but remember that spinach wilts as it cooks.
Return the pan to the oven. Bake the frittata until it's puffed and a toothpick inserted in its center comes out clean, 25-30 minutes.
Cool the frittata for 15 minutes in the pan before slicing it into eight triangles and serving.
You can probably use frozen spinach, but I haven't tried it so I can't give you exact directions. I would use frozen whole spinach leaves, defrost them in the microwave, and VERY THOROUGHLY drain them. A 10-oz package of frozen spinach is the equivalent of about 1 lb. of fresh spinach. So for this recipe, I assume you would need about 3-4 ounces of frozen spinach.