How to Make It
Peel onion, and cut vertically into 8 wedges, keeping root end intact to hold wedges together. Combine harissa and 1 tablespoon oil in a 5- to 6-quart slow cooker. Add potatoes, carrots, and onion; toss gently to coat.
Combine garlic powder, paprika, pepper, and 1/4 teaspoon salt in a small bowl. Sprinkle mixture evenly over meaty side of chicken thighs. Heat a large skillet over medium-high. Add remaining 1 tablespoon oil to skillet; swirl to coat. Add chicken thighs, meaty side down. Cook until well browned on one side, 5 to 6 minutes.
Arrange chicken thighs over vegetables in slow cooker, browned side up. Cover, and cook on low 8 hours. Remove chicken from slow cooker, and sprinkle vegetables with remaining 3/4 teaspoon salt. Divide vegetables evenly among 4 bowls; top evenly with chicken. Spoon juices over chicken and vegetables. Serve with lemon wedges and garnish with parsley, if desired.