Start by preheating your oven to 425°F and lining a large baking sheet with parchment paper (or just brush the pan with oil).
Next, add the chicken thighs, halved potatoes, and red onion to a large bowl. Pour in the olive oil, tomato paste, and lemon juice, then add the smoked paprika, cumin, coriander, turmeric, cinnamon, kosher salt, and black pepper. Toss until everything is evenly coated.
Arrange the chicken in a single layer on the baking sheet, then place the potatoes and onions around it. Make sure nothing is piled on top of each other. If the pan looks crowded, spread things out so everything roasts instead of steams.
Transfer the pan to the oven and bake uncovered for 30 minutes, checking at the 20-minute mark. The chicken is done when it reaches an internal temperature of 165°F, and the potatoes should be fork-tender with golden, crispy edges.
While everything roasts, make the creamy lemon tahini sauce. In a small bowl, whisk together the Greek yogurt, tahini, lemon juice, lemon zest, grated garlic, and kosher salt. Then add cold water a little at a time, whisking as you go, until the sauce is as thick or as drizzly as you like.
Once the chicken and potatoes are done, pull the sheet pan out of the oven. Drizzle the sauce over the top (or serve it on the side), garnish with fresh parsley, and enjoy!
Store leftovers in an airtight container in the fridge for up to 3 days, and keep any extra sauce in its own container.