Whisk the eggs: In a bowl, whisk eggs with milk or water, salt, and a few grinds of pepper until smooth and slightly frothy.
Preheat the pan: Warm a nonstick skillet over medium heat. Add olive oil and let it shimmer.
Soften the veggies: Add red onion and bell pepper.
Cook 2–3 minutes until slightly tender. Stir in tomatoes and cook 1 minute more. Transfer vegetables to a plate.
Cook the eggs: Reduce heat to medium-low.
Pour in the eggs and tilt the pan to spread evenly. As the edges set, gently push them toward the center with a spatula, letting uncooked egg flow to the edges.
Add the filling: When the top is still a little glossy but mostly set, sprinkle oregano, return the warm vegetables, add spinach, feta, and olives (if using) over one half.
Fold and finish: Fold the omelette over the filling. Cook 30–60 seconds more until the cheese softens and the eggs are just set.
Avoid browning.
Garnish and serve: Slide onto a plate. Top with parsley or dill, a crack of pepper, and a light squeeze of lemon for brightness.