Heat a skillet over medium heat and cook the ground turkey until it is no longer pink, breaking it up as it cooks. Drain any excess liquid and transfer the turkey to the slow cooker.
Add the rinsed lentils, diced onion, diced carrots, minced garlic, harissa paste, diced tomatoes, chicken broth, cumin, salt, and pepper to the slow cooker.
Stir everything together until the harissa and spices are evenly distributed.
Cover and cook on low for 7 to 8 hours or on high for 4 hours, until the lentils are tender and the stew has thickened.
Taste and adjust salt or pepper if needed. Ladle into bowls and top each serving with a spoonful of Greek yogurt and a sprinkle of chopped cilantro.