Make the onion jam. Heat the oil in a saucepan and gently fry the onions, thyme sprigs and some salt and pepper for 20—25 minutes until really soft and golden. Discard the thyme sprigs and stir in the sugar, vinegar and redcurrant jelly. Simmer for 6—8 minutes until the sauce is thick and jam-like, then leave to cool completely.
Brush the mushrooms with the oil and season with salt and pepper. Cook on a hot barbecue for 2—3 minutes on each side. Meanwhile, toast the ciabatta rolls for about 1 minute on each side until lightly browned.
Fill each roll with two mushrooms and some spinach leaves and spoon in some onion jam and aïoli, if liked. Serve at once.