I recommend watching this video by Kenji López-Alt if you are making Spanish omelette for the first time! He explains it well and gives a nice overview of how to make it properly.

https://www.youtube.com/watch?v=RPYk9W9v-bI

Ingredient List

• [ ] 150ml olive oil
• [ ] 1 large yellow onion
• [ ] 450g royal trumpet or enoki mushrooms
• [ ] 240g baby spinach
• [ ] 5 whole chicken eggs
• [ ] Salt, to season
• [ ] Black pepper, to season

Instructions

1. Finely slice the onion and mushrooms, and roughly chop the spinach.
2. Using a frying pan on low-medium heat, cook the spinach with 1 tbsp of olive oil until most moisture is removed from the spinach, about 2-3 minutes. Keep aside once cooked.
3. Using a frying pan on low-medium heat, cook the spinach with 1 tbsp of olive oil until most of the moisture is removed from it, about 2-3 minutes. Once cooked, keep aside.
4. Heat a large nonstick fry pan with medium heat and add in ~10 tbsp (150 ml) olive oil.
5. Add in the onions and cook on low-medium heat and stir occasionally until slightly translucent for about 5 minutes. Meanwhile, crack the eggs into a large mixing bowl and season with salt & black pepper, whisk together.