Place chicken in a large Ziploc bag. Drizzle with olive oil. Season liberally with salt and pepper. Add oregano, basil, thyme, and garlic to the bag. Seal the bag, toss to coat the chicken. You can let the chicken marinate in the refrigerator for a couple of hours (or overnight), or you can just use it right away.
Place onion and carrots in the bottom of a slow cooker. Top with chicken and herbs from the bag. Squeeze lemon juice over top.
Cover and cook on LOW for about 4-6 hours, or on HIGH for about 2-3 hours.
Remove chicken to a plate, bowl, or cutting board. Chop or shred the meat, then stir it back into the pot. Taste and season with salt and pepper, if desired.
Use a slotted spoon to pile the chicken on top of pita bread, rice, or salad. I typically discard the carrots in the pot, which were just used to flavor the meat as it cooked. Garnish the chicken with Tzatziki sauce, feta cheese, and other toppings of choice.