Put the chillies in a mortar and, using a pestle, pound until ground.
Put the ground chillies, whisked yoghurt, garlic and ginger pastes and dried mint into a blender or food processor, then season with salt and process to make a smooth paste.
Transfer to a large shallow dish, add the chicken and, rub the paste evenly all over to coat. Cover and allow to marinate in the refrigerator for 1 hour.
Heat a little oil in a non-stick pan over medium heat, add the chicken and cook for about 5 minutes on each side, or until cooked.
If you prefer to steam the chicken, place the chicken in a clean banana leaf square, fold it over to make a parcel, and secure it with cocktail sticks (toothpicks).
Use a large pan with a lid and place a trivet or upside-down plate or small bowl in the base. Pour in enough water so it stays below the top of the trivet or plate and cover with the lid. Place the banana leaf on a plate and carefully put on top of the trivet, plate, or bowl, cover with a lid, and steam for about 10 minutes, or until the chicken is cooked.
Sprinkle some green chillies in vinegar and a little dried mint on top of the chicken and serve with a slice of lemon and onion rings.