Put the onion and stock in a heavy saucepan, cover with a lid and bring to the boil.
Boil for 5 minutes, then uncover the pan and simmer gently over a low heat for about 25 minutes, until the onions are tender and golden and all the liquid has been absorbed.
Strain off any liquid that has not evaporated.
Meanwhile, cook the potatoes in lightly salted boiling water until tender.
Drain, remove the skins and slice thinly into rounds.
Break the eggs into a bowl and beat lightly with a fork.
Add some salt and pepper, and then stir in the cooked onion and potatoes and the chopped parsley.
Heat a seasoned or non-stick 20cm frying pan and spray the base and sides with low calorie cooking spray. Pour the egg mixture into the hot pan and reduce the heat to a bare simmer. Cook very gently over the lowest possible heat for about 15 minutes, or until the underneath is set but the top is still slightly runny.
Place the pan under a preheated hot grill, just long enough to set and lightly brown the top of the omlette.
Slide the omelette out on to a serving plate. Cut into wedges and serve with a crisp salad.