Place the orange on a chopping board and use a small sharp knife to cut off the top and bottom so that you cut right through the peel and outside pith to the flesh. Now cut the remaining peel and pith away from the flesh, cutting in strips downwards, following the curve of the orange. Cut the orange into fleshy segments, using a sharp knife to carefully cut either side of the inside pith. Discard the peel and pith, keeping only the segments.
Slice or shred the cooked chicken thighs, discarding the bones and skin, if preferred. Divide the watercress between 2 plates and arrange the orange segments, chicken and sliced avocado attractively over the watercress. Drizzle with walnut oil and scatter over a few walnut pieces, if desired.