I use another 'Delia' method.
Firstly, pour some oil in a frying pan that's about the size you want the omelette to be.
Then, get the oil as hot as you possibly can, and ensure you circulate it around so that the edges of the pan have an oil coating.
Add your whisked-up omelette mix.
Now here's the trick. Once the eggs hit the pan, they start cooking. Holding onto the pan handle, get a tablespoon in your other hand. Hold it by the left edge of the omelette and tilt the pan towards your left. The liquid part of the the mix runs down to fill the space while the spoon holds the cooking part in place. Repeat for the right hand side and just keep doing this in any direction. The result is you end up with a perfectly circular omelette with a a cooked top, and because you've been gently moving the mix around, the bottom isn't burnt.
Then usually on one half I'll add some grated cheese and maybe some cooked bacon or ham. Use a fish slice to fold the omelette into a perfect half-moon and then lift out of the pan and serve.
Perfect shape, perfectly cooked through, not a single burn. By doing this, you can cook an omelette in less than 2 minutes.