Slow Cooker Greek Chicken Gyros with Tzatziki

• 2 pounds boneless skinless chicken breast
• 1 cup diced onion, about 1 medium onion
• 2 teaspoons minced garlic 
• 1 teaspoon dried oregano
• ½ teaspoon salt
• ½ teaspoon ground black pepper
• 3 tablespoons olive oil
• 1 tablespoon red wine vinegar
• 2 tablespoons lemon juice
• ⅓ cup water

Tzatziki Sauce

• 2 cups plain greek yogurt
• 1 cup finely grated cucumber
• 2 teaspoons minced garlic 
• 1 tablespoon white wine vinegar
• 1 teaspoon dried dill
• 1 teaspoon dried oregano
• 2 tablespoons lemon juice
• 1 tablespoon extra-virgin olive oil
• Salt, to taste
• ground black pepper, to taste

For Serving

• Warm pita bread
• diced red onion
• shredded romaine lettuce
• diced cucumber
• diced tomatoes