'Mille-feuille' literally means 'thousand leaves' and is a type of cake, sometimes known as a custard slice, though these days you can see a variety of flavours. On the savoury side there is also the salmon millefeuille which consists of layers of smoked salmon between bread, crackers or pastry with cream and other ingredients involved. The leaves refer to multiple layers though there will always be much fewer than a thousand.
Mille-feuille is also an approach to making an omelette and one that received attention at the time of the salmonella in UK eggs scare of 1988, when people were warned that 'runny' eggs could contain the contamination and the British took to cooking all their eggs very thoroughly. My mother adopted the approach of grilling her omelettes after cooking them in a pan and continues to do so today even 28 years later. Grilling an omelette can help crisp out the outer layer and give it the golden brown colour. However, if you are using decent quality eggs as outlined in my previous omelette posting and you put a splash of olive oil in with your cooking butter in the pan, then you should get this anyway.
The concern to cook every egg thoroughly and not have uncooked beaten egg liquid in the centre of your omelette leads to mille-feuille. It also works well if you are cooking an omelette for a lot of people who want the same filling or no filling. You are replacing breadth for depth. You make one large omelette and then simply slice it. This will look odd to some people but is the easiest way when cooking using this approach. I have done it with nine eggs in the liquid (I saw 'liquid' rather than 'mixture' because as I noted in the first posting of this series there should be nothing in there that was not in the egg, i.e. milk), without difficulty.
Rather than drawing the egg into the middle as it cooks and filling the emptied space of the pan with more egg mixture or using a large pan simply allowing the omelette to rise, instead you keep folding. Once you have covered the whole pan once, you fold a half of the cooked egg over and then fill the remaining half with egg liquid. Once this new half is well on the way to cooking you fold over the first half into it. This exposes the other half of the pan and you fill this with more egg mixture. Once this is on its way to cooking, you bring the now fatter half back over on to it.
You can repeat this halving and turning for as long as you have egg liquid. Typically I will do the process three or four times. When you cut into the omelette you will see that there are a number of layers, perhaps six or eight, maybe more. Thus, it tastes different on the tongue to a 'standard omelette'. In fact you are eating a series of nested omelettes. The important thing for the British is that each layer is thin, not thick as if you had poured all the egg liquid in at once. This means it will be cooked through. There is an additional benefit if you are looking to include fillings.
One of the greatest errors with omelette fillings is to put too much in. I heard on Radio 1 recently a DJ had tweeted to the world to ask what he was doing wrong with his omelettes. The renowned chef Tom Kerridge tweeted back that his filling to egg liquid mixture was too high. People stuff their omelettes and then break up the structure far too much. They wonder why they end up with a 'mess' but this is always going to be the case if they overload. The thing to remember is that whilst the omelette looks big and robust, in fact it is the weakest element. Ham and mushrooms, even some herbs are heavier than even the cooked omelette and can easily tear through the omelette structure. Cheese, as I have noted before, is different and can work as a bond between components of the omelette.
Now, when using the mille-feuille approach to omelettes this is no warrant to go mad with your fillings. However, because of the layering, if you get them in early in the folding process, even if they fragment the inner layers, there will be the outer ones coming along to seal over any gaps and package up the whole thing. With mille-feuille do not leave fillings until too late, start getting them in on the first or second layer. You can put cheese into the outer layers without risk and if you are making a particularly thick omelette then this can help you.
The main challenge with the mille-feuille approach to omelettes is getting the halves over neatly without breaking up the omelette structure. You need good tools for this and something which is firm not a flimsy plastic turner but something rigid and broad enough to carry the bulk of the omelette even when it has grown. I have done it with a flat knife and a fork, but something broader is better, even a fish slice! The other thing is to watch the heat. People forget that even if the electricity or gas is kept at a steady level, the pan you are using is getting hotter and hotter. Thus, as you get into the middle and outer layers, take the heat down; gas is better for this than electricity, but remember even lifting the pan off the heat, the omelette will keep cooking from the heat that is already in the metal. There is nothing worse than a burnt outer layer. For the reason also keep butter standing by in case there is a need to stop the liquid adhering to the pan as you continue.
This is a straight forward approach to large omelettes or ones with a good deal of fillings to be contained in them or for people who are squeamish about getting any drop of uncooked egg in their omelette.