You will also need quite a lot of time. Not that you will be spending hours slaving over a hot stove, but there's some long, slow cooking with occasional stirring involved, so keep this to make on a wet afternoon when you don’t feel like going out. Pop a good film on the TV and remember to give it a stir in each commercial break.
For the filling you will need
1kg onions, peeled and roughly chopped
6 sprigs of thyme
100ml cold water
3 large or extra large eggs
50ml double cream
50g Emmenthal cheese, roughly grated
Place the onions, water and leaves from the thyme in a heavy, lidded non stick pan. Bring to the boil then reduce the heat to very low and cook for about 2 hours, until the onions are extremely soft and just about melting, stirring frequently to make sure they don’t catch. Keep an eye on the liquid and add more if necessary.
When they are really soft, increase the heat to high and continue cooking, stirring very frequently, until all the water has been driven off. Stir in the butter and reduce the heat to medium. Continue cooking for 10 minutes until you have a rather unappetising looking brown mush like that in the photo below. During this 10 minutes, stir a few times but do allow a few places to catch slightly to give a hint of “fried onion” taste.
Heat the oven to 220C, 200 fan, 450 F, gas mark 8. While it is heating, beat together the eggs, cream, a little salt and a good grinding of black pepper. Stir this into the onion mixture along with about half the grated cheese, turn the whole lot into the flan case and level the top, then sprinkle over the remaining cheese.
Bake for 15 minutes then reduce the oven temperature to 180C, 160 fan, 350F, gas mark 4 and continue cooking for 45 minutes to an hour until set and browned. If it starts to look too brown before it is fully cooked, drape gently with a sheet of greaseproof paper.
Best allowed to stand for half an hour before eating, so it is tepid rather than hot.
Serve with green salad and a few new potatoes. Hazelnut mayonnaise ( use your favourite mayonnaise recipe replacing half the olive oil with hazelnut oil) makes a delicious salad dressing to serve with this.