Season both sides of chicken with salt and pepper.
Place the chicken in the bottom of the slow cooker, overlapping only as much as necessary.
Top with onions, peppers, mushrooms, crushed tomatoes, garlic, oregano, olives and wine. Give it a quick stir.
Cover and cook on LOW for 4-6 hours or until an instant-read thermometer inserted into the chicken reads 165°F (74°C). You can also cook it on HIGH for 2-3 hours. (Boneless chicken breasts need 3 hours on Low or 1 ½-2 hours on High.)
When the chicken is done cooking, transfer to a serving platter and cover with aluminum foil.
Optional: You can thicken the sauce by adding the mixture of 1 tablespoon of cornstarch mixed with 2 tablespoons of water to the crock pot. Cover and cook on high for 15 minutes. (To thicken faster, cook it in a skillet on medium heat for 3-5 minutes.)
Taste the sauce and add more salt if desired.
Serve on top of pasta and spoon extra sauce over the chicken. Garnish with optional parsley or herbs of your choice.