In a bowl, mix the lemon zest and juice with the oil and sugar, season with salt. Add the celery, shallot and garlic to the dressing, mix and set aside.
Rinse the sea bream (inside and out) in cold water, pat dry. Make crosswise incisions (approx. 5 mm deep) on both sides of the fish without damaging the bones. Season the fish inside and out, brush with oil.
Brush the rhubarb stems with oil, grill over/on a medium heat (approx. 200 °C for approx. 3 mins. all over. Grill the fish with the lid down for approx. 6 mins. on each side. Cut the rhubarb into slices, mix into the salsa with the basil, mix with the sea bream.