Heat a large nonstick fry pan (I used an 11 inch / 32 cm pan) with a medium heat and add in 3 tbsp (45 ml) extra virgin olive oil

While the oil is heating, roughly chop the onion, then add into the pan with the hot olive oil, mix together

Meanwhile, cut the mushrooms in half and roughly chop the spinach

After 6 to 7 minutes and the onions are lightly sauteed, add in the mushrooms, mix together, then place the mushrooms in a flat layer and cook without mixing for 1 minute, then mix and once again place in a flat layer and cook for 1 minute, continue to cook the mushrooms in this technique

Once the mushrooms are lightly sauteed, about 5 to 6 minutes, add in the chopped spinach, add in batches to not overcrowd the pan, mix the spinach with the mushrooms so it easily wilts, once all the spinach has been added, season everything with sea salt & black pepper, mix together and turn off the heat

Crack the eggs into a large bowl, season with sea salt & black pepper and whisk together, then add in the mushroom and spinach mixture, mix together and let it sit for a couple of minutes

Meanwhile, heat the same pan with a low-medium heat and add in 1 tbsp (15 ml) extra virgin olive oil

After a couple of minutes add in the egg mixture, make sure it´s all in a flat layer, after a couple of minutes run a spatula through the outer edges to ensure the omelette is not sticking to the pan

About 4 to 5 minutes after adding the egg mixture, place a plate that is a bit smaller than the pan over the pan, in one swift move flip the pan into the plate, then slide the omelette back into the pan to cook the other side, using the back of the spatula, compact the omelette from the outer edges towards the center, this gives it that classic rounded edge, cook for another 3 to 4 minutes, then remove from the heat

Slide the omelette into a serving dish, serve warm, at room temperature or even chilled, enjoy!