Preheat oven to 180°C fan forced. Lightly grease a 7cm deep, 10cm x 20cm (base) loaf pan. Use the prosciutto to line base and sides of the pan allowing a little overhang along both long edges.
Heat the oil in a large non-stick frying pan over medium heat. Add onion and cook for 3 minutes until soft. Add mushrooms, increase heat to high and cook 4 minutes until mushrooms are soft and most of the moisture evaporated. Set aside to cool.
Place the couscous into a heatproof bowl, pour over the boiling water. Cover with a plate and stand for 3 minutes. Remove the cover, stir gently with a fork to separate the grains.
Combine the mushrooms mixture, couscous, chicken mince, basil, egg, currants and capsicum in a large bowl. Season with salt and pepper and mix until well combined. Spoon the mixture into the greased pan. Use your hands to press down firmly and smooth the surface. Fold the prosciutto over chicken loaf. Bake for 40-45 minutes or until cooked when tested with a skewer. Set aside in the pan for 10 minutes before turning out and slicing. Serve with salad or vegetables.