In a large bowl combine the red onion, coriander and cumin.
Add the avocados to the bowl and mash well with a fork. Add the chillies, lemon and season generously and mix well.
Place the guacamole in the centre of 4 bowls, spoon the sour cream on top and finally sprinkle with the chives.
Pile the tortillas around the edge of the bowls and serve immediately.
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