Prep the cucumber yogurt. Grate the cucumber on the large holes of a box grater. Sprinkle with a pinch of salt and let it sit for 5 minutes. Squeeze out excess liquid with your hands or a clean towel.
In a bowl, combine the cucumber with yogurt, half the garlic, half the lemon juice, a pinch of salt, and half the herbs. Stir and set aside in the fridge.
Make the marinade. In a separate bowl, whisk yogurt, olive oil, remaining garlic, lemon zest, remaining lemon juice, cumin, coriander, paprika, red pepper flakes if using, 1 teaspoon salt, and black pepper.
Marinate the chicken. Pat the chicken thighs dry. Add them to the marinade and coat well.
Cover and refrigerate for at least 30 minutes, ideally 2–4 hours. Overnight is great if you have time.
Preheat and prep. Heat your oven to 425°F (220°C). If you want extra-crisp skin, set a wire rack inside a rimmed baking sheet and place the chicken on the rack. Otherwise, use a lightly oiled sheet pan.
Shake off excess. Lift each thigh from the marinade, letting extra drip off.
Too much yogurt can burn. Arrange thighs skin-side up on the pan. Lightly season the tops with a pinch of salt and pepper.
Roast to juicy perfection. Roast for 30–40 minutes, depending on size, until the skin is golden and crisp and the internal temperature reaches 175°F (80°C).
For deeper color, broil for 1–2 minutes at the end. Rest the chicken for 5 minutes.
Finish the sauce. Stir the chilled cucumber yogurt. Add remaining herbs and adjust salt, pepper, and lemon to taste.
For extra richness, drizzle in a teaspoon of olive oil.
Serve. Spoon cucumber yogurt onto plates, top with chicken, and add any pan juices. Garnish with more herbs and lemon wedges. Serve with warm flatbread, rice, or a simple salad.