In a large saucepan, melt the butter and add the sliced onions, garlic and sugar.  Cook on a fairly high heat until starting to colour then reduce the heat down to the very lowest possible on your stove and leave to cook for at least 30 minutes until they are almost on the point of collapse.  The bottom of your pan will be covered in brown and caramelised gunk - this is ideal, it will taste delicious later on.

Pour in the stock, Marmite and cider, scrape the bottom of the pan to dislodge the sticky goo (see above), turn up the heat, and once it has come to the boil leave to simmer gently without a lid for 45 minutes to an hour until reduced.  This can be done well in advance and just reheated when you want to serve it.

For the croutons, preheat the grill to its highest setting.  Place 6 slices of bread under the grill, (and keep an eagle eye on them as they turn to charcoal in seconds when your back is turned).  Turn them over to toast both sides.

In a bowl, combine the egg yolk, cheese, spring onions and cider and spoon this over each slice of toast, drizzling any liquid remaining in the bowl over the top.

Toast under the grill until golden brown and bubbling.  As this was a main course, I served a greedy 3 croutons per person, but if its a starter one will suffice!