In a large bowl, whisk together the Greek yogurt, harissa paste, lemon juice, and minced garlic. Add the chicken thighs, ensuring they are fully coated, and marinate for at least 30 minutes.
Preheat the oven to 400°F (200°C).
Toss the halved baby potatoes with olive oil, dried oregano, salt, and pepper on a rimmed baking sheet. Arrange in a single layer with the cut side down and roast for 25 to 30 minutes until the skins are crispy and the centers are tender.
Heat a cast-iron skillet or grill pan over medium-high heat. Remove chicken from the marinade, shaking off excess, and sear for 6 to 8 minutes per side until the internal temperature reaches 165°F (74°C).
In a small mixing bowl, combine the crumbled feta, sour cream, chopped dill, and lemon zest. Use a fork to mash the ingredients into a thick, slightly chunky cream.
Allow the chicken to rest for 5 minutes before slicing. Serve the chicken atop or alongside the crispy potatoes and finish with a generous dollop of the dill feta cream.