Drizzle the olive oil into a small frying pan over a medium-high heat and fry the chopped onion for 5 minutes, until soft.
Pour the milk into the pan and crack in the egg. Quickly stir the ingredients in the pan together with a fork, making a scrambled egg and onion mixture. Make sure you crack the yolk of the egg so that you get scrambled eggs and not an ordinary fried egg!
Season with salt and pepper and carry on stirring for 2-3 minutes until you've got light and fluffy scrambled eggs.
Whilst the egg is cooking, sprinkle the cheddar cheese in a thick line down the centre of the tortilla and place the chopped ham on top.
Layer the cooked scrambled egg-onion mixture on top of the ham and top with the diced avocado.