Slice the yellow onion into thin rings and set aside. Rinse the tomatoes and cucumbers, then dice the tomatoes and slice the cucumbers. Gather all fresh toppings in bowls.

In a small bowl, combine minced garlic, chicken broth, olive oil, dried oregano, ground paprika, salt, and black pepper. Whisk until well-blended.

Layer the sliced onions at the bottom of the slow cooker. Place the chicken breasts on top and pour the seasoning mixture over the chicken.

Cover the slow cooker, set it to low, and cook for 6-8 hours, or on high for 3-4 hours until the chicken shreds easily.

Remove the chicken from the slow cooker and shred using two forks. Mix the chicken back with the onions and juices in the slow cooker.

Warm the pita bread. Fill each pita with shredded chicken and onion, then top with tzatziki sauce and preferred veggies.