Cut the skin and the white membrane off the oranges. Working over a bowl to catch the juice, cut between the membranes to remove the segments. Slice the avocados and toss gently with the orange segments. Pile on to serving plates.
Reserve a few whole cardamom pods for garnishing. Crush the remainder using a mortar and pestle to extract the seeds or place them in a small bowl and crush with the end of a rolling pin. Pick out and discard the pods.
Mix the seeds with the oil, honey, allspice and lemon juice. Season to taste and stir in the reserved orange juice. Garnish the salads with sprigs of watercress and the reserved cardamom pods and serve with the dressing spooned over the top.