Spray slow cooker with nonsitck cooking spray.
Place bell pepper and onion in bottom. Add chicken and drizzle with olive oil.
Sprinkle evenly with greek seasoning and salt.
Cover and cook on high 1 3/4 hours or until chicken is no longer pink (vegetables will be slightly crisp)
Remove chicken and slice. Remove veggies with a slotted spoon.Sauce
Combine yogurt, mayo, mustard, and 1/4 tsp salt in a small bowl. Whisk till smooth.
Warm pitas according to pacakge. Cut in half and layer chicken, sauce, veggies, and feta. Top as desired.