Bring a medium saucepan of salted water to the boil for the chicken. Halve the unpeeled potatoes, quartering any large ones. Put in a separate medium saucepan, cover with cold salted water and bring to the boil. Reduce the heat to medium and cook for 10-12 mins until tender. Drain.
Step 2 — Prep chicken
While the potato is cooking, slice the chicken on an angle into 4 equal pieces.
Step 3 — Poach chicken
Add the chicken to the pan of boiling water, then immediately reduce the heat to very low. (Make sure the water is barely simmering while cooking the chicken to prevent it from drying out.) Cook, covered, for 6-7 mins until the chicken is cooked through. Remove from the pan with a slotted spoon, lightly season with salt and pepper and set aside to cool.
Step 4 — Prep horseradish dressing
Meanwhile, finely chop the capers. Combine the capers, horseradish dressing and 2 tsp water in a large bowl and season with salt and pepper.
Step 5 — Make rocket salad
Halve the cucumber lengthwise and thinly slice. Thinly slice the radish. Whisk 2¼ tbs extra virgin olive oil, 3 tsp white wine vinegar and 2 tsp dijon mustard in a large bowl and season with salt and pepper. Add the cucumber, radish and rocket to the vinaigrette and toss to combine.
Step 6 — Make chicken salad
Thinly slice the spring onion. Thinly slice or shred the chicken. Add the chicken, spring onion and potato to the horseradish dressing, season with salt and pepper and toss to combine. Divide the rocket salad and chicken salad among bowls and drizzle with any remaining dressing to serve.