One factor that determines whether an omelette is made cigar or flat round in shape is the amount of garnish to be added. A flat round omelette will hold much more than a folded one.
When making flat round omelettes the garnish is generally added to the mixture before cooking. Follow the Method for Plain Omelette up to Stage (4) then continue:
(5) When set and firm enough, toss like a pancake and immediately transfer it to a warm, buttered dish. Alternatively, lightly set the top exposed area under a salamander grill. This takes only a few moments, the length of time being highly critical as even a second or two too long will make the omelette hard, dry and rubbery.
(6) Transfer the cooked omelette to a plate or round buttered flat dish, bottom side uppermost.
(7) Brush the surface with melted butter and complete with the appropriate garnish items.