Gather all your tools and ingredients, and preheat your non-stick skillet over medium-low heat until warm.
In a small bowl, whisk the eggs until just combined, creating a smooth, slightly frothy mixture.
Roughly chop the spinach, then squeeze out any excess water to prevent sogginess in your omelette.
Add a teaspoon of olive oil to the skillet. Once shimmering and just starting to crackle, add the sliced mushrooms.
Sauté the mushrooms over medium heat for 2-3 minutes until they turn golden and smoky, stirring occasionally.
Add the prepared spinach to the pan, sauté for 1-2 minutes until wilted and bright green, then remove from heat.
Pour the beaten eggs evenly over the mushroom and spinach mixture in the skillet. Shake gently to distribute the ingredients.
Cook over medium-low heat for about 3-4 minutes until the edges are golden and the top is just starting to set but still slightly jiggly in the center.
If using cheese, sprinkle it over one half of the omelette, then fold gently with a spatula to enclose the filling.
Let the omelette rest off the heat for 30 seconds to finish setting, then slide onto a plate.
Garnish with chopped fresh herbs and a crack of pepper, then serve immediately while warm and tender.
