What is the rule regarding raw fillings like un-sautéed onions in omelette preparation?

All vegetables must be fully cooked, or at least warm or at room temperature, before adding.

The cooking timeline for an omelette is extremely short, often lasting only a few minutes from the time the eggs hit the hot pan until the folding process begins. Consequently, any fillings that are not pre-prepared cannot cook sufficiently. Raw vegetables, such as onions or mushrooms, will remain crunchy within the finished product. More critically, raw produce releases internal moisture, or weeps, when heated. This released moisture mixes directly into the delicate egg base, effectively diluting the structure, making it runny, and severely increasing the risk of structural failure when attempting the final fold or roll.