Chop the spinach into bite-sized pieces and slice the mushrooms thinly; set aside.
Beat the eggs in a small bowl until smooth and slightly frothy, seasoning with a pinch of salt and pepper.
Heat the skillet over medium heat, then add the oil or butter, swirling to coat the surface until shimmering.
Add the minced garlic to the pan, cooking for about 30 seconds until fragrant—listen for a gentle sizzle and notice the aroma filling the kitchen.
Stir in the sliced mushrooms, cooking for 2-3 minutes until they turn golden brown and release a rich, earthy scent.
Add the chopped spinach to the pan, stirring quickly to wilt and soften, which takes about 1-2 minutes; the greens will turn vibrant and slightly shrunk.
Pour the beaten eggs evenly over the mushroom and spinach mixture, tilting the pan to distribute the eggs uniformly. Let it cook undisturbed for 1-2 minutes until the edges start to set and turn a light golden color.
Reduce heat to low, then sprinkle the grated cheese over one half of the omelette. Cover the pan with a lid and cook for another 2-3 minutes until the top is just set and the cheese has melted into the eggs.
Gently fold the omelette in half using a spatula, pressing lightly to seal in the fillings. Let it cook for another 30 seconds to warm through.
Slide the omelette onto a plate, resting for a minute so the flavors settle, then slice and serve immediately, enjoying the tender eggs with crispy edges and flavorful fillings.
