Toast the spices in a dry frying pan over a medium heat, until fragrant (but not burning), about 2-3 minutes. Grind the spices using a pestle and mortar or a spice grinder.
Add the spices to a large bowl with the ginger, yoghurt, olive oil and salt, mix well.
Add the chicken thighs and lemon slices, massage the yoghurt marinade into the meat. Cover and leave to marinate for at least 2 hours, but ideally overnight, in the fridge.
Before you want to cook the chicken, remove it from the fridge about 2 hours before, and allow it to come to room temperature. Preheat the oven to 200°C.
Place the chicken on a roasting tray, with space in-between each thigh — you don't want to overcrowd the tray. You may need to use two roasting trays for this. Drizzle with olive oil and roast in the oven for 40-45 minutes until the chicken is golden brown, and cooked through.