Prep the spinach filling. Heat a small nonstick skillet over medium heat; add a tiny drizzle of olive oil. Add garlic and onion (if using); cook 30 seconds. Add fresh spinach; sauté 1–2 minutes until fully wilted and any released water has evaporated. Season with salt and pepper. Transfer to a small bowl and wipe the skillet clean.

Whisk the eggs. Crack 3 eggs into a bowl. Add 1 tbsp water or milk, salt, and pepper. Whisk vigorously for at least 1 minute until the mixture is pale, frothy, and full of air bubbles — this is the key to a fluffy omelette. The more air you whisk in, the lighter the result.

Heat the pan. Return the clean skillet to medium heat. Add butter or olive oil; tilt the pan to coat the entire surface evenly. The pan is ready when the butter foams but has not yet browned.

Cook the eggs. Pour the egg mixture into the pan all at once. Let it sit undisturbed for 20–30 seconds until the edges begin to set. Using a rubber spatula, gently push the cooked edges toward the center while tilting the pan so the uncooked egg runs to the edges. Repeat until the eggs are mostly set but the top still looks slightly glossy and wet — about 2–3 minutes total.

Add the filling. Spread the cooked spinach evenly over one half of the omelette. Sprinkle the cheese on top of the spinach. Add any fresh herbs now.

Fold and serve. Fold the plain half of the omelette over the filled half. Slide onto a plate immediately. The residual heat will finish melting the cheese inside. Garnish with fresh chives or parsley and serve right away.