Preheat the oven to 180°C/Fan 160°C/Gas 4. Tip the pearl barley into a pan and cover with boiling water. Add a pinch of salt. Simmer for 40 mins till tender. Rinse under cold water.
Put half the raspberries into a bowl. Add 1 tbsp Balsamic vinegar and 1 tbsp olive oil. Crush with a fork to make a dressing. Season.
Finely chop the mint, reserving any small leaves to garnish. Finely slice the fennel. Halve the cucumber. Slice one half in half lengthways and scoop out the seeds. Chop into small cubes.
Make a few slashes in each duck breast with a sharp knife. Rub with a little salt and pepper. Heat a non-stick frying pan till smoking hot. Place the duck breasts in the pan skin side down. Cook for 2 mins each side.
Place the duck breasts on a baking tray, skin side up. Cook for 8 mins for medium, 10 mins for well done. Pop the duck on a warm plate. Cover with foil and rest for 10 mins.
Pour the duck juices into the raspberry dressing and whisk together. Tip the cooled pearl barley into a large bowl. Stir through most of the dressing. Add the rocket, fennel, cucumber and mint. Season and toss.
Slice the duck breasts at an angle. Divide the pearl barley salad between two plates. Top with the duck breasts. Drizzle over the raspberry dressing. Scatter over the reserved raspberries and mint leaves.