Via: Dan Jones
Put the vinegar and 100ml water in a pan with the sugar, mustard seeds and star anise. Heat gently to dissolve the sugar, then bring to the boil. Remove from the heat to cool slightly for 5 minutes.
Put the radishes, fennel and mange tout in a bowl. Add the warm pickling liquid and set aside.
Cook the potatoes in boiling salted water for 20 minutes until tender. Drain, halve and tip into a bowl.
Heat a frying pan over a medium high heat. Brush the tuna steaks with a little oil and season, then sear for 1-2 minutes each side or until cooked to your liking. Rest briefly before slicing.
Discard the star anise, drain the pickled vegetables and toss with the potatoes and the olive oil. Add all the herbs to the bowl and mix gently. Serve with the seared tuna.