Put the chicken between 2 sheets of parchment paper or cling film and bash with a rolling pin to flatten. Grate the zest from ½ orange and mix in a bowl with the mustard, 2 tsp oil and plenty of seasoning. Add the chicken and stir well.
Heat a griddle or frying pan and cook the chicken for 5-6 mins on each side until cooked through. Place on a plate to rest while you assemble the salad.
Tip the salad leaves into a bowl. Peel and slice the oranges on a plate to catch any juices. Pour the orange juice over the leaves, add the slices with 1tsp oil and toss together. Add the beetroot, then slice the chicken and add to the salad along with any resting juices. Divide everything between plates and serve.