Add the sliced onion and minced garlic to the bottom of the slow cooker to create a base layer.
Place the chicken thighs on top of the onion and garlic.
In a small bowl, stir together the harissa paste, olive oil, salt, black pepper, and chicken broth until combined.
Pour the harissa mixture over the chicken, then scatter the sliced roasted red peppers on top.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours until the chicken is tender.
Serve the chicken with the peppers and sauce spooned over it, then sprinkle with the chopped fresh parsley.