Heat the oven to 180°C/350°F/gas mark 4.
Place the celeriac chunks in a baking tray, season with salt and pepper and sprinkle over 2 tablespoons of the olive oil, making sure the celeriac is nicely coated. Place the celeriac in the oven and roast for about 30 minutes, then remove the tray from the oven and carefully arrange the plum halves in and around the chunks. Tuck in the sage and rosemary, and trickle everything with the remaining olive oil. Return the tray to the oven, and continue roasting for 15–20 minutes, until the plums are lovely and soft, but still holding a little shape. Keep your eye on them as plums can cook at different rates depending on how ripe they are.
When you’re happy with how everything is looking, remove the tray from the oven and scatter over the blackberries. Taste the celeriac and plums, seasoning with salt and black pepper, and trickling over a little honey to sweeten, if necessary.
Transfer the salad to a bowl or platter and serve straight away.
