Pat chicken dry. Toss with salt, pepper, smoked paprika, oregano, and cumin.
Warm olive oil in a skillet over medium-high heat. Sear chicken 2–3 minutes per side.
Add onion, bell peppers, and garlic to the slow cooker. Pour in crushed tomatoes and broth. Stir in red pepper flakes.
Nestle the seared chicken into the sauce and spoon some over the top.
Cover and cook on Low 4–5 hours or High 2.5–3.5 hours, until the chicken is tender.
Stir in olives and sherry vinegar. Cook 15 minutes to meld flavors.
For a thicker sauce, mix cornstarch with water. Stir into the cooker and cook 5–10 minutes.
Taste and adjust seasoning. Serve hot with rice, potatoes, or crusty bread.