Set the oven to 200°C (400°F). Oil a baking dish, about 20 x 20 cm (8 x 8 inches), and line a rimmed sheet with parchment paper.
Toss the celery root cubes with the oil and salt, spread them in a single layer on the sheet, and put them in the oven. Set a timer for 12 minutes to turn them.
Heat 1 tablespoon of the olive oil in a skillet over low heat. Add the breadcrumbs and stir constantly for 2 to 3 minutes, until golden and fragrant. Tip into a bowl.
Squeeze the soaked raisins dry and chop them. Add them to the breadcrumbs with the pine nuts, sugar, parsley, orange juice, remaining oil, salt and pepper. Mix until the filling looks like damp, loose sand.
Spoon about a tablespoon of filling onto each fillet. Roll it up from the head end toward the tail.
Set the rolls snugly in the dish, tail side up, tucking bay leaves between them if using. Scatter the leftover filling on top and drizzle with oil.
Slide the dish into the oven beside the celery root, which by now has been roasting for 10 to 15 minutes. Bake the sardines for 20 to 25 minutes, until the top is golden and crisp.
Push a thermometer into the thickest roll. It should read 63°C (145°F).
Check the celery root. It should be fork-tender and golden at the edges. If it needs more time, return only the sheet to the oven.
Let the sardines cool for about 10 minutes. Serve on the celery root with lemon wedges and parsley.