Preheat the grill to high. Use 1 tablespoon of the olive oil to grease the grill tray. Place the trout fillets on the tray, skin-side down, and scatter over the lemon zest. Season with black pepper and grill for 8-10 minutes, until cooked.
Meanwhile, bring a pan of water to the boil and add the runner beans and green beans. Cook for 5-6 minutes, drain and leave to cool slightly, then transfer to a bowl. Stir in the spring onions and drained butterbeans.
Whisk together the mustard, honey, mint, lemon juice and remaining 1 tablespoon olive oil. Season with black pepper and stir into the beans. Serve with the trout and a wedge of lemon, and garnish with mint leaves.