Preheat oven to 230°C (210°C fan-forced). In the centre of a heavy rimmed baking tray, toss celery, onions and garlic with 1 tbs oil to coat. Coat chicken all over with 2 tsp olive oil. Season with salt and pepper and arrange over vegetables on the tray.
Roast the chicken for 50 mins or until an instant-read meat thermometer inserted in the thickest part of the thigh registers 73°C, adding the bread, brushed with 1 tbs olive oil, to the tray for the last 8 mins of cooking.
Transfer the chicken and focaccia to a serving platter. Set the chicken aside for 15 mins to rest.
Meanwhile, in a large saucepan over medium heat, add enough vegetable oil to come 5cm up the side of the pan. Heat to 175°C. Carefully add one-quarter of the brussels sprouts to pan. Quickly cover with a lid to contain any oil splatter. Uncover after 5 secs and cook for 2½ mins or until crisp and golden brown. Using a slotted spoon, transfer the brussels sprouts to a baking tray lined with paper towel. Season with salt. Repeat, in 3 more batches, with the remaining brussels sprouts, reheating the oil to 175°C between each batch.
Grate lemon rind and parmesan over the brussels sprouts. Cut the lemon into wedges and squeeze over the brussels sprouts. Serve immediately with the chicken and focaccia.