Place stock in a large saucepan and add potatoes, carrot and celery. Bring to the boil then reduce heat and simmer until vegetables are tender, about 20 minutes.
Remove from heat, add baby spinach and use a stab blender to process mixture until smooth. Season to taste with salt and freshly ground black pepper.
Meanwhile for aioli, blanch Broccolini® in boiling salted water till tender, about 2 minutes. Cool in icy water, drain, and place into a food processor. Pulse until roughly chopped and transfer to a bowl. Fold in mayonnaise, garlic and pistachios. Set aside in a cool place.
Return blended vegetables to the saucepan over medium to low heat. Add prawns, diced salmon and Broccolini® stems and simmer for about 5 minutes or until seafood is cooked and stems are tender.
Add Broccolini® florets and remove from heat. Stand until Broccolini® is warmed through. Stir through chives and parsley.