Preheat the oven to 200 degrees C. Place the tomatillos and chillies into a small roasting pan or a rimmed baking tray. Roast for about 20 minutes, turning frequently. If the chillies start to get too dark, remove them.
Heat the oil in a large frypan over medium-high heat. Add chicken pieces and cook, turning to sear evenly on all sides.
Combine the tomatillos and chillies in a blender or food processor and blend until smooth. Season with salt to taste. Pour this mixture in with the chicken. Reduce the heat to medium-low, cover and simmer until chicken is tender, 20 to 25 minutes.