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Bring a large pan of water to the boil, add the broccoli and cook for 2 mins. Drain, then refresh under cold water. Heat a griddle pan, toss the broccoli in 1⁄2 tsp of the rapeseed oil and griddle for 2-3 mins, turning, until a little charred. Set aside to cool. Brush the chicken with the remaining oil and season. Griddle for 3-4 mins each side or until cooked through. Leave to cool, then slice or shred into chunky pieces.
Next, make the pesto. Pick the leaves from the basil and set aside a handful to top the salad. Put the rest in the small bowl of a food processor. Scoop the flesh from the avocado and add to the food processor with the garlic, walnuts, oil, 1 tbsp lemon juice, 2-3 tbsp cold water and some seasoning. Blitz until smooth, then transfer to a small serving dish. Pour the remaining lemon juice over the sliced onions and leave for a few mins.
Pile the watercress onto a large platter. Toss through the broccoli and onion, along with the lemon juice they were soaked in. Top with the beetroot, but don’t mix it in, and the chicken. Scatter over the reserved basil leaves, the lemon zest and nigella seeds, then serve with the avocado pesto.