Place half of the roasted red bell peppers, the tomato sauce, dried oregano, dried basil, sugar, and crushed red pepper flakes into a small food processor or blender and puree until smooth.
Season the chicken thighs with the Kosher salt and cracked pepper, then arrange them skin side up in the slow cooker.
Tuck the remaining roasted red peppers, the kalamata olives, and the artichoke hearts around the chicken pieces in the slow cooker.
Pour the pureed tomato-and-pepper sauce evenly over the chicken and around the edges of the slow cooker.
Cook on high for 4–5 hours or on low for 7–8 hours, until the chicken is cooked through and tender.
Carefully remove the chicken and serve with the peppers, olives, and artichokes spooned over the top.