Put the chives, basil and mint in a small food processor with the lemon zest and juice, oil and mustard. Blitz to a purée. Take out 2 tbsp to use as a coating for the fish, then stir the capers and anchovies into the rest.
Cook the runner beans in boiling salted water or steam for 5 mins until tender but still with a little bite.
Brush the tuna with the reserved herby mixture and griddle for 2-3 mins each side until cooked but still a little pink in the centre. Toss half the caper dressing through the warm beans with the parsley and pile onto plates. Top with the tuna, spoon over the remaining dressing and scatter with the almonds and mint leaves.