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.
Video
Notes
Variations:
If you don't have fresh onions on hand, you can use ½ teaspoon of onion powder instead.
You can also substitute sliced green onions for the onion. I like the color that green onions add to the frittata.
If you don't mind adding meat, try mixing in a handful of bacon crumbles or a cup of chopped cooked chicken breast. I especially like bacon. It greatly enhances the flavor of the frittata.
You can use crumbled feta cheese instead of parmesan. The flavor profile will be different, but just as good.
Leftovers:You can keep the leftovers in the fridge, in an airtight container, for up to 4 days. Reheat them in the microwave at 50% power or enjoy them cold.