Add in flour to the eggs and whisk until it is well mixed.
In a frying pan heat up the oil.
Cook and stir the vegetables until they start to soften, add longer cooking vegetables first so the soft ones don’t overcook.
Pour in the batter and lower heat to a minimum. When the eggs set around the edges flip the pan and cook the other side for 3 minutes, or until it sets.
I have found that using a double-sided omelette pan (Amazon affiliate link) makes this incredibly easier. I found mine at the local Nacional supermarket.To make it in the oven just oil the pan (I like individual serving ones, but use what you have), arrange your vegetables in it, pour the egg mixture and pop in the preheated oven at 200 ºF [100 ºC] until eggs have set and it is light-golden on top (you can pinch it with a toothpick to check for doneness).