Slice up all the onions. I tend to slice them into half rings when doing this rather than the usual little chopped pieces, don’t know why, it just seems right!
Put them in a frying pan with 50g butter, yes, I know it’s a lot, but it is one of the main flavourings. Saute the onions for about 30-45 minutes, until they are a dark brown colour. Stir every now and again in case they start to stick, if they do, add just a splash of water
Once the onions are cooked, leave them to cool until warm, if not completely cold.
Whisk the eggs, milk and mustard together and add to the cooled onions. The onions need to be cool or they will scramble the eggs
Bake
Bake at 180°C Fan / 200°C / 400°F / Gas Mark 6 for about 30 minutes until the filling is cooked through and firm