Crack the eggs into a small bowl and whisk until the yolks and whites are just combined, creating a smooth, slightly frothy mixture.
Heat the skillet over medium heat and add a tablespoon of olive oil, letting it shimmer gently with a light ripple sound.
Add the sliced mushrooms to the skillet and sauté, stirring occasionally, until they turn golden brown and release their earthy aroma—about 3-4 minutes.
Push the mushrooms to one side of the skillet, then add the spinach. Stir and cook until wilted and vibrant green, about 1-2 minutes, filling the kitchen with fresh, leafy scent.
Pour the beaten eggs evenly over the mushrooms and spinach, allowing them to settle into the pan. Let cook undisturbed for about 20 seconds until the edges just start to set and bubble slightly.
Gently tilt the pan and use the spatula to lift the edges, letting the uncooked egg flow underneath. Continue this process for about 1-2 minutes, until the surface is mostly set but still slightly creamy on top.
If desired, sprinkle cheese or fresh herbs over one half of the omelette now for added flavor, then cover the pan briefly to help melt the cheese, about 30 seconds.
Once the top is just set and the bottom has turned a beautiful golden brown, carefully fold the omelette in half with the spatula or slide it onto a plate, letting it rest for a minute to settle and finish cooking through.
Finish with a pinch of salt, pepper, and additional herbs if using. Serve immediately, preferably hot, with toast or fruit for a complete breakfast.
