1️⃣ Sauté the vegetables
Heat the olive oil or butter in a nonstick skillet over medium heat until warm but not sizzling. Add the sliced mushrooms and cook for 3–4 minutes, stirring occasionally, until softened and lightly golden. Add the spinach and cook just until wilted, about 30–60 seconds. Transfer the vegetables to a plate and set aside.

2️⃣ Whisk the eggs
Crack the eggs into a bowl and whisk with a pinch of salt and freshly ground black pepper until fully combined and slightly frothy.

3️⃣ Cook the eggs low and slow
Reduce heat to medium-low. Pour the eggs into the same skillet. Let them cook gently, lifting the edges with a spatula and tilting the pan so the uncooked eggs flow underneath. Cook until mostly set with a slightly glossy surface on top.

4️⃣ Fill and fold
Spoon the mushroom-spinach mixture over one half of the omelet and sprinkle with shredded Gruyère. Use your spatula to gently lift and fold the omelet in half.

5️⃣ Finish and serve
Cook for another 1–2 minutes, just until the cheese melts and the center is set but tender. Slide onto a plate and serve immediately.