Place the olive oil and the sage leaves into a fry pan and heat up, until the sage leaves turn crisp. Season the tuna on all sides then place into the oil. Grill for 1 minute only on each side then remove and set aside. Keep a pink seam visible in its centre.
Combine the squashed potatoes with the pesto and place onto four plates. Combine the radish slices, fennel and parsley with the mandarin oil and place onto the plates.
Place the tuna onto the potatoes and garnish with the cress leaves