Preheat oven to 180°C. Place a wire rack in a large roasting pan. Combine the breadcrumbs, fruit and nut mix, prosciutto, egg and spring onion in a bowl. Pat the chicken cavities dry with paper towel. Spoon stuffing loosely into the cavities. Use kitchen string to tie the legs together. Place chickens on the rack in the pan. Sprinkle with sea salt flakes. Drizzle with oil.
Roast, brushing chickens occasionally with chilli jam, for 11/2 hours or until juices run clear when thickest part of thighs are pierced with a skewer. Cover and set aside for 10 mins to rest.
Meanwhile, place the egg yolks in a heatproof bowl over a saucepan of simmering water (don’t let bowl touch the water). Whisk for 5-8 mins or until pale. Gradually add the butter, whisking after each addition, until the mixture is thick and well combined. Stir in the lemon juice and chopped rosemary.
Transfer the chickens to a serving platter. Sprinkle with rosemary sprigs and serve with the sauce.