Heat 2 tablespoons of olive oil in a large sauté pan over medium-high heat. Once hot, add the boneless skinless chicken thighs and sear for about 4-5 minutes on each side until golden brown. Transfer to the slow cooker.

In the same pan, add the sliced sweet onion and minced garlic to the remaining oil. Sauté for approximately 4-5 minutes until the onion is tender and translucent. Transfer the mixture to the slow cooker over the seared chicken.

Add the roasted red peppers, olives, chicken broth, capers (if using), dried oregano, dried thyme, fresh rosemary, and bay leaf to the slow cooker. Season with salt and pepper, ensuring everything is evenly distributed.

Cover the slow cooker with its lid and set it to low heat. Cook for about 4 hours, monitoring to ensure even cooking. The chicken should become fork-tender.

After 4 hours, check the doneness of the chicken; it should easily shred apart. Adjust seasoning with extra salt and pepper if necessary. Before serving, squeeze fresh lemon juice over the top.