Pat 6 chicken thighs dry with paper towels. Removing excess moisture will ensure the skin browns nicely. Season the chicken with 2½ tsp. salt.
Cook chicken thighs (skin side down) in a dry large cast-iron skillet on a gas stove top on a medium heat until the skin is crispy and brown. Do not turn, touch or move the chicken. Leave it for about 15 minutes and when it moves easily off the base of the skillet, you know it is crispy. If you need to ensure even distribution of heat, move the skillet over the flame- not the chicken around the skillet!
Transfer the chicken skin side up, to a plate using tongs.
In the same skillet pan, add the cherry tomatoes, harissa paste, red wine vinegar, 1 sprig of oregano and remaining ½ tsp salt. Increase the heat to medium-high and simmer, stirring occasionally until the tomatoes have burst and released all of their juices. The juices will thicken. This should take about 8–10 minutes.
Gently place the chicken thighs back into the skillet, placing them skin side up.
Reduce the heat to medium-low, simmering until the flesh of the chicken is no longer pink and the sauce has thickened. This should take 6–8 minutes.
Switch the heat off and allow the chicken to rest for 5 minutes. This allows the flesh to reabsorb the moisture, making it tender and juicy. Once the chicken has rested, roughly crumble the feta over the casserole. Remove the leaves off the remaining 2 oregano sprigs and sprinkle over the top of the dish as a finishing garish.
Serve with cheesy breadsticks to lap up any extra tomato sauce.
Serving Size 1