Chop onions into quarters and slice (easy in a food processor)
Melt the butter & oil into the bottom of the slow cooker
Pile all the onions in with plenty of freshly ground pepper
Cook on high for about an hour stirring now and then to get the onions all covered by the butter & oil
Reduce the temp to low and cook for 12+ hours - I have difficulty resisting the temptation to keek peeking, smelling and stirring, and it doesn’t seem to matter!
The onions should be a rich golden-brown at this point
Add the stock, vinegar, fish sauce & garlic
Cook on low for 6-8 hours minimum, the longer you cook it the deeper the flavour
Add the brandy if using, half an hour or so before serving - add salt now according to taste
Optional - I like to give the soup a few quick blitzes with my Bamix (wand-style liquidiser), but don’t overdo it, you want the onion slices!
— To Serve
Option 1:
Ladel soup into bowls
Top the French toasts with grated gruyere cheese; float them on top of the soup and bake or grill (broil if you’re American!) till the cheese bubbles
Option 2 (if you don’t trust your soup bowls to cope with option 1)
Top the toasts with gruyere and grill (or broil) till cheese bubbles
Ladel the soup into bowls and float the cheesy toasts on top
Enjoy - and wait for the compliments!