Cajun chicken traybake with sweet potato wedges chive dip

Name: Cajun chicken traybake with sweet potato wedges & chive dip
Ingredients: 1 tsp each of ground cumin and smoked paprika; 1 tsp dried oregano; 1 tbsp fresh thyme leaves or 1 tsp dried; 8 skinless, boneless chicken thighs (900g); 1 red and 1 yellow pepper, deseeded and chopped; 3 red onions, sliced into 1cm-thin wedges; 2 celery sticks (125g) , finely chopped; 1 tbsp rapeseed oil, plus 1 tsp; 2 sweet potatoes (375g peeled weight), cut into 16 wedges; 150g Kefir or bio yogurt; 2 tbsp snipped chives