4 large onions
320ml dry sherry, such as fino
200ml vegetable stock, hot For the stuffing
75g butter, plus extra to dot
60g crustless sourdough bread, whizzed into crumbs
120g mixed nuts (we used almonds, hazelnuts and macadamias), pulsed briefly in a food processor
5-6 fresh thyme sprigs, leaves picked
2 fat garlic cloves, finely chopped
100ml dry sherry
2 large free-range egg yolks