In a small food processor, blend the basil, mint, spring onion greens, lemon zest, juice, oil, and mustard into a purée. Reserve 2 tbsp for coating the fish, then mix in the capers and anchovies with the rest.
Boil or steam the runner beans in salted water for 5 minutes until tender but slightly firm.
Boil new potatoes until cooked, drain and add a splash of olive oil, salt, pepper and a garnish of spring onion. Shake the pan with the lid on until the potatoes start to break apart ~5seconds.
Brush the tuna with the reserved herb mixture and grill for 2-3 minutes per side, keeping the center slightly pink. Toss half the caper dressing with the warm beans and parsley, plate, and top with the tuna. Drizzle with the remaining dressing, then sprinkle with and mint leaves. Serve and enjoy!