What seems to work best for me is a tip from Bengali Bob's: cook it for as long as the recipe calls for and leave it resting overnight. The onions (and other veg) will continue cooking until the heat dissipates and the fibres will have tenderised further. This is particularly good if you cooked in a pressure cooker, as it keeps the heat for longer, but I reckon BB does not use one. It took me 3-4 mins whizzing it with a stick blender vs my norm of 15-20mins til right consistency.