Mix all the ingredients for the marinade in a bowl, add the chicken breasts and coat well on all sides. Refrigerate for at least an hour or more.
Heat the oven to 180℃. Line a baking sheet with parchment paper, Toss the grapes in 1½ tsp olive oil and salt. Spread the grapes on the prepared baking sheet and roast for 15 minutes till the grapes are lightly golden and a little shrivelled in appearance.
Place the balsamic vinegar in a saucepan and bring to a boil. Turn down the heat and simmer, stirring intermittently till it has reduced by nearly half to a syrupy consistency. Cool.
Heat a pan on low heat and toss in the walnuts. Toast them for a couple of minutes, taking care not to burn them. Cool.
Heat 1 tbsp olive oil in the same pan on medium heat. Place the chicken breasts and let them char and cook for 3-4 mins on each side undisturbed till they're cooked through. Their internal temperature should be 74℃. Remove from the pan, cool a little and slice.
To construct the salad, place the chicken slices in a single layer on a platter, followed by the cucumbers. Scatter the the grapes and walnuts around evenly, followed by the rocket. Lightly sprinkle a little salt and freshly ground pepper to season the salad. Finally, drizzle the salad with the balsamic reduction and serve.