Pat the chicken thighs dry and rub with a pinch of salt, pepper, and a light dusting of cumin and paprika for an extra layer of flavor. This step helps create a flavorful crust on the thighs as they brown, even though they’ll finish cooking in the slow cooker.
In a skillet over medium heat, warm 1–2 tablespoons of olive oil. Sear the chicken thighs skin-side down for 3–4 minutes until the skin turns a rich golden brown. Flip and sear the other side for 1–2 minutes. Transfer the thighs to the slow cooker.
In the same skillet, add the remaining oil then sauté the onions until translucent, about 3–4 minutes. Add the minced garlic and cook for another 30 seconds until fragrant.
Stir in the diced tomatoes, chicken broth, cumin, coriander, paprika, cinnamon, turmeric, and red pepper flakes. Bring to a light simmer, scraping up any browned bits from the pan to deepen the sauce.
Slice the sweet potatoes into roughly 1-inch chunks. Layer them around and on top of the chicken in the slow cooker. Pour the tomato-spice sauce over everything, and scatter the olives on top.
Cover and cook on low for 6–7 hours, or on high for 3–4 hours, until the chicken is tender and the sweet potatoes are easily pierced with a fork.
Check the seasoning and adjust with salt and pepper to taste. Just before serving, stir in chopped cilantro or parsley for a bright finish.
Serve hot, spooning the saucy potatoes and chicken onto bowls. A squeeze of lemon or a dollop of yogurt can add a fresh contrast if you like.