Clean, wash and cut the celery into small pieces. Blanch in boiling salted water for about 4 minutes, then drain. Wash the apple, cut 2 slices from the middle to garnish.
Quarter the remaining apple, cut out the core. Dice flesh and sprinkle with lemon juice. Clean and wash the rocket and cut it into bite-sized pieces. Wash liver, dab dry and cut into smaller pieces.
Fry in hot oil while turning for about 4 minutes. Season with salt and pepper. Melt the fat. Steam celery and apple pieces for 3-5 minutes. Fold in rocket. Season with coarsely ground pepper. Serve everything garnished with prepared apple slices and parsley.