Heat a large, dry nonstick frying pan until hot, add the pumpkin seeds and dry-fry for 3–5 minutes, stirring frequently, until golden and starting to pop. Leave to cool.
When cool, place the pumpkin seeds in a food processor or blender with the onion, chilli, oregano, coriander and 75 ml (3 fl oz) of the stock and whizz to a smooth paste.
Heat 1 tablespoon of the oil in a saucepan, add the paste and cook for about 10 minutes, stirring frequently, until thickened. Pour in the remaining stock and simmer for 15 minutes.
Meanwhile, rub the remaining oil over the chicken, then season. Heat a griddle pan until smoking hot, add the chicken and cook for 5–7 minutes on each side until just cooked through. Cut into slices and arrange on a serving plate with the radishes. Serve with the sauce spooned over.