Finely slice the onions (a mandolin kitchen gadget works well here).
Fry the onions in olive oil over medium heat.
Sprinkle with salt and stir often for 15 minutes. This slow cooking releases the onion's natural sugar.
Add the balsamic vinegar and stir for another 2 minutes.
Stir in the flour, then slowly whisk in the chicken stock until the mixture has a gravy consistency.
If you're a lump-free gravy fan, consider investing in a hand blender to blitz the mixture. Leave the gravy in the pan until you're ready to eat so you can quickly re-heat it just before serving. If it's too thick, just add a little water.