Rinse the spinach and place it in a large skillet over medium-low heat. Cover and steam for a few minutes until wilted and soft, stirring occasionally.
While the spinach is steaming, start preparing the rest of the tortilla ingredients. Add the flour, sourdough discard, oil, and salt to the bowl of a stand mixer fitted with a dough hook.
When the spinach is soft, remove it from the pan along with any remaining water, and place it in a food processor or blender and blend until smooth. You can also just finely chop it with a chef's knife on a cutting board.
Add the spinach, along with any remaining water/juices into the flour mixture. With the dough hook and a low-medium setting (setting number 4 on my mixer), knead the dough for 3 minutes until smooth. While it kneads, sprinkle in small amounts of flour as needed to keep the dough from sticking to the sides. Alternatively, you can knead by hand for 5 minutes.
You can now let your dough rest, covered, for 30 minutes to 2 hours. Or if you are in a hurry, they do work to roll out right away. The timing is pretty flexible.
When you are ready to cook your tortillas, divide the dough into 12 equal parts. Shape each piece into a ball and roll out as thin as you can into a circle, sprinkling flour on your work surface as needed to keep the tortilla from sticking.
Lightly grease a cast iron skillet with avocado oil and preheat it on medium-low. Cook each tortilla for 30 seconds to 1 minute on each side, until light bubbles form.
Transfer each the tortilla to a platter and cover with aluminum or a towel to keep warm until ready to use.