Place the sliced green, red, and yellow bell peppers and sliced onion in the bottom of the slow cooker.
In a small bowl, mix together the smoked paprika, dried oregano, ground cumin, salt, black pepper, and crushed red pepper flakes.
Rub the spice mixture evenly over the chicken breasts.
Heat the olive oil in a skillet over medium-high heat. Sear the chicken breasts for 2 minutes on each side until lightly browned.
Transfer the seared chicken breasts to the slow cooker, placing them on top of the vegetables.
Add the minced garlic and chicken broth to the slow cooker.
Cover and cook on low for 4 hours or until the chicken is cooked through and tender.
Once cooked, remove the chicken breasts and stir the vegetables to combine with the cooking juices.
Drizzle fresh lemon juice over the chicken breasts before serving.
Serve the chicken breasts with the cooked peppers and onions.