Step 1: Pat the chicken thighs dry with paper towels. Season generously with salt and pepper.
Step 2: (Optional, but recommended for flavor) Heat olive oil in a large skillet over medium-high heat. Sear the chicken thighs for 3-4 minutes per side until golden brown. You may need to do this in batches. Transfer the seared chicken to the slow cooker insert.
Step 3: In the same skillet (if searing), add the chopped onion and red bell pepper. Sauté for 5-7 minutes until softened. Add the minced garlic and cook for another minute until fragrant.
Step 4: Stir in the harissa paste, coconut milk, diced tomatoes (undrained), cumin, and smoked paprika into the skillet with the vegetables. Bring to a gentle simmer, scraping up any browned bits from the bottom of the pan.
Step 5: Pour the sauce and vegetable mixture over the chicken thighs in the slow cooker. Ensure the chicken is mostly submerged in the sauce.
Step 6: Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, until the chicken is very tender and easily shreds with a fork.
Step 7: Carefully remove the cooked chicken thighs from the slow cooker and place them on a cutting board. Using two forks, shred the chicken into bite-sized pieces.
Step 8: Return the shredded chicken to the slow cooker. Stir in the Greek yogurt until well combined and the sauce is creamy. Taste and adjust seasoning if needed (more salt, pepper, or harissa).
Step 9: Serve the Slow-Cooker Creamy Harissa Chicken hot over couscous, rice, or with warm naan bread. Garnish with fresh cilantro or parsley, if desired.