Yes, you absolutely can. I sometimes have leftover spinach puree in my freezer and one cube works great in this recipe. You can also just take a handful of frozen spinach from the bag. Then, defrost the spinach, and try to get most of the excess moisture out of it with a paper towel. Last step is to add the spinach into the omelette along with the feta cheese. No need to saute it like you would the fresh spinach.