Preheat oven to 450F
Marinate the chicken in greek yogurt, garlic, 1/2 the spice mix and lemon juice. Set aside.
Line a sheet pan with parchment paper and scatter red onion slices on top.
In a large bowl toss together chicken breasts, the rest of the spices, sweet potatoes, and olive oil.
Spread the seasoned chicken and sweet potatoes evenly over the onions. Try to keep everything in a single layer for even roasting. Transfer the sheet pan to the oven and bake for 25–30 minutes. For extra color and crispness, switch to the broiler for the final 5 minutes.
For the yogurt sauce: Drain your cucumber. After shredding, squeeze it out with a paper towel or cheesecloth to avoid a watery sauce. In a bowl, mix together the yogurt, cucumber, garlic, salt, olive oil, and lemon juice. Let chill in the fridge for about 10 minutes before serving.
Place the sheet pan on the table and let everyone dig in. Serve with warm pita, yogurt sauce, shredded lettuce, sliced tomatoes, pickled turnips, and red onions. If you prefer a rice bowl, just swap the pita for a scoop of rice and build your bowl your way.