1Combine paprika, cumin, coriander, cayenne, salt, and black pepper in a small bowl.
2Heat 2 tbsp olive oil in a skillet over medium-high heat. Add the onion slices and cook until soft and starting to caramelize, about 5 minutes. Transfer to a plate.
3Add remaining 1 tbsp olive oil to the skillet. Stir the garlic into the spice mix, then add the chicken pieces. Sprinkle the spice mixture all over and toss to coat evenly.
4Cook the chicken, stirring occasionally, until golden and cooked through, about 12–14 minutes.
5Stir in the lemon juice and cooked onions, cooking for 1 more minute to let the flavors marry.
6Meanwhile, stir the cilantro and a pinch of salt into the yogurt.
7Warm the pita bread (wrap in foil in a 300°F oven for 5 minutes, or warm directly over a flame if you have a gas stove).
8Assemble: spread cilantro yogurt on each pita, add chicken and onions, then top with cucumber, tomato, and red pepper.