Prep the egg whites: Add egg whites to a bowl with a pinch of salt, pepper, and the tablespoon of water or milk if using.
Whisk until frothy and slightly airy, about 20–30 seconds.
Heat the pan: Place a nonstick skillet over medium heat and add the olive oil. Swirl to coat. If using garlic, add it now and cook for 20–30 seconds until fragrant, not browned.
Wilt the spinach: Add the spinach to the skillet.
Cook, stirring, for 1–2 minutes until just wilted. Season lightly with salt and pepper.
Pour and set: Spread the spinach evenly in the pan. Pour in the egg whites.
Tilt the pan to distribute the eggs evenly around the spinach.
Control the heat: Reduce to medium-low. Use the spatula to gently pull the edges toward the center while tilting the pan so uncooked egg flows to the edges. Do this for about 1–2 minutes.
Add herbs: Sprinkle the fresh herbs and a pinch of red pepper flakes over the top.
Finish cooking: When the top looks slightly glossy but mostly set, stop stirring.
Cover the pan for 30–45 seconds to gently steam the top and finish cooking without browning.
Fold and serve: Slide the spatula under one side and fold the omelette in half. Transfer to a plate. Add one more crack of black pepper and enjoy right away.