Heat your oven to 200°C/Fan 180°C/Gas 6. Cut the leaves and stalks from the beetroot and carrots. Set them to one side. Scrub the veg. Cut the beetroot into 1cm thick slices. Then cut them into 1cm wide lengths. Cut the carrots in half lengthways.
Lightly crush 2 tsp coriander seeds in a pestle and mortar. Place the veg on a baking tray. Drizzle over 1 tbsp olive oil. Scatter over the coriander seeds with some salt and pepper. Toss everything together. Drop on the garlic cloves, unpeeled. Roast for 40 mins, till the veg is tender and a little charred.
Tip the pearl barley into a pan. Cover with 500ml boiling water. Pop on a lid. Simmer for 20 mins till the barley is just tender. Drain and leave to cool.
Peel and thinly slice the onion. Zest and juice the orange and lemon into a bowl. Add 2 tbsp honey and season. Whisk together. Add the onion and toss to coat. Leave to pickle.
When the veg has cooked for 30 mins, set a griddle or frying pan on a high heat. Toss the chicken fillets with ½ tbsp olive oil, salt and pepper. Place them in the pan and cook for 4 mins on each side till cooked through. Scatter over the cumin seeds and cook for a further 1 min. Place them on a plate.
Halve the avocados and scoop out the flesh. Wash the beet and carrot leaves well. Finely shred a small handful of each. Spoon the onions into the pan with the barley, leaving most of the citrus liquid in the bowl, and stir together. Add the beet and carrot leaves. Taste and season.
Pluck the garlic from the veg tray and squeeze out the flesh. Stir it into the reserved citrus marinade. Add the chicken fillets and stir to coat them.Scatter over the lemon zest.
Arrange the watercress on a serving platter or a couple of plates. Pile on the onion barley, roasted veg and the chicken. Pour over the citrus dressing. Finish with the avocado halves.