*Celery is listed as moderate (polyol) thus you should not exceed 1 stalk as a portion.
Preheat the oven to 240C/475F/Gas 9. Begin by removing the tough outer layers of the celery, then pare the outside of the root off, but leave it attached. Now cut across the celery about 9cm/3½in from the base. Stand the lower half upright and cut vertically through the centre.
Then cut each half into 4 to make 8 pieces, keeping them attached to the root. Save a couple of nice leaves (preferably attached to a small stem) and trim the top pieces of celery to a similar length to the base, cutting off any really tough and stringy edges. Now wash all the pieces and dry them on kitchen paper.
Next heat 1 tablespoon of the oil in a frying pan, then lightly brown the celery, keeping them on the move so they brown evenly. Now transfer them to a plate. Increase the heat under the pan, add the pancetta and fry the slices until they're really crisp - it will take 2-3 minutes and you'll need to keep turning them.
Next lay the silicone paper over the baking tray and lightly grease a circle of 23cm/9in on it. Arrange the celery in an attractive shape on the paper, putting the prettiest pieces on the top, add the thyme, rosemary and sage leaves in among it, and season with salt and pepper.
Now combine the remaining olive oil, tomato puree and wine vinegar, sprinkle that over the vegetables, followed by the pancetta crumbled into pieces with your hands. Next fold the silicone paper over and seal, making pleats, all round - you may find a couple of metal (not plastic) paper clips useful here, as it's essential to keep the steam trapped inside. Place the parcel in the pre-heated oven for 20-25 minutes.
After that carefully unwrap the paper - you may need scissors - and serve the vegetables with the juices spooned over.