I only do cheese ones generally but DH and DD1 (the only ones who eat them apart from me) like them:
2-3 eggs, add small amount of water, beat roughly and add seasoning
heat lump of butter in small omelette pan until hot and frothy but not burning; pour eggs in, swirl round pan, sprinkle grated cheese on top immediately
keep cooking over high heat, pushing cooked edges of egg into middle and tipping pan around so runny bits roll into hot spaces
keep cooking until it's as runny/set as you want then tip onto plate, folding second half over first half.
They're not proper French type omelettes, which are practically liquid ugh, but taste fab.
Apparently it is v important never to wash an omelette pan - just wipe it out with paper each time.