Layer the base: Add the sliced onion and red bell pepper to the bottom of the slow cooker. Scatter in the garlic, cherry tomatoes, sun-dried tomatoes, olives, and capers.
Season the chicken: Pat the chicken dry.
Season both sides with salt, pepper, oregano, thyme, and paprika. Place the chicken on top of the veggies.
Add liquid and brightness: In a small bowl, whisk olive oil, chicken broth, lemon juice, and lemon zest. Pour the mixture over the chicken and vegetables.
Sprinkle on the red pepper flakes if using.
Set and cook: Cover and cook on Low for 5–6 hours or High for 2 1/2–3 1/2 hours, until the chicken is tender and reaches 165°F internally. Thighs can go longer without drying out.
Optional finish: For a thicker sauce, remove the chicken to a plate and tent with foil. Turn the slow cooker to High, and simmer the sauce uncovered for 10–15 minutes, or stir in a cornstarch slurry (1 teaspoon cornstarch + 1 teaspoon water) and cook 5 minutes.
Taste and adjust: Stir, then taste the sauce.
Add a pinch more salt, a squeeze of lemon, or extra olives if you like it brinier.
Serve: Return the chicken to the pot, top with chopped parsley or basil, and crumble feta over the top if desired. Serve with couscous, rice, or crusty bread, plus a simple green salad.