Crack the eggs into a small bowl, season generously with salt and pepper, and whisk until just combined, aiming for a light, slightly frothy mixture.
Heat the skillet over medium heat and add the olive oil or butter, allowing it to shimmer or melt with a gentle crackle.
Add the sliced mushrooms to the hot pan, sauté for 2-3 minutes until they turn golden brown and release their earthy aroma, stirring occasionally.
Toss in the drained spinach and cook for about 1 minute until wilted and fragrant, stirring gently. Remove the vegetables from the pan and set aside.
Pour the beaten eggs evenly over the hot skillet, tilting the pan to spread the mixture into an even layer. Let it cook gently on medium-low for 2-3 minutes until the edges begin to set and turn a light golden.
Scatter the sautéed mushrooms and spinach over one half of the eggs, and sprinkle with cheese if using. Continue cooking until the top is mostly set but still slightly runny, about 1-2 minutes.
Carefully fold the omelette in half with a spatula, pressing gently to seal the filling inside. Cook for another 30 seconds to melt the cheese and finish setting the eggs.
Gently slide the omelette onto a plate, sprinkle with fresh herbs, and let it rest for a minute to settle. Slice and serve warm, enjoying the fluffy texture and savory filling.
