Tip the beans into a large roasting tin, add the cherry tomatoes, onion and some seasoning and mix together. Slash each side of the trout two or three times with a knife. Tuck the bean mixture in between the slits.
To make the pesto, finely chop the rocket leaves and pine nuts and pound in a pestle and mortar. Alternatively, blitz in a blender or food processor. Mix with the Parmesan, oil and some seasoning. Spoon a little of the pesto into the cuts in the trout and the inside of the fish, then spoon the rest of the mixture into a small bowl. Cover both and chill until required.
When ready to serve, remove the cover and roast the trout in a preheated oven, 190°C (375°F), Gas Mark 5, for 20–25 minutes depending on their size. Test by pressing a knife into the centre of the trout through the body cavity: if the fish flakes evenly and is an even colour it is ready. Transfer to serving plates and serve with spoonfuls of pesto and extra rocket, if liked.