Preheat the Blackstone griddle to medium heat and lightly grease with butter or oil.
Crack the eggs into a bowl, add milk (or water), salt, and pepper, then whisk until frothy.
Pour the egg mixture onto the griddle and spread into a circle with a spatula.
Once the eggs begin to set but are still slightly soft on top, sprinkle your desired fillings over half of the omelette.
Fold the omelette in half with a spatula and cook for 1–2 more minutes until the cheese melts and the eggs are cooked through.
Slide onto a plate and serve warm.