Spinach and corn work well together because they bring contrast to the eggs in both flavour and texture.
Spinach softens quickly and blends into the omelette with a mild, savoury taste, while sweet corn keeps a little bite and adds natural sweetness.
Many omelette methods recommend preparing fillings before the eggs go into the pan, and spinach especially benefits from a quick sauté to cook off moisture. That helps the omelette cook more evenly while keeping the filling flavourful and the texture light.