You can check if the omelette is done by poking into it with a chopstick.
If you pull out the chopstick and it is clean your omelette is done. If there are moist residues from the eggs on the chopstick the omelette needs a few more minutes.
I used frozen spinach that I thawed in the microwave before adding it to the pan.
If you use fresh spinach you don’t necessarily need to precook it, but give it a few more minutes of frying in the pan.
I like to eat this dish with some cream cheese on the side.
You may also add shredded cheese on top right before you put the pan into the oven.