Heat the olive oil in a large pan over medium high heat. Season the chicken on both sides with salt and pepper.
Cook the chicken for 4-5 minutes per side, or until golden brown.
Remove the chicken from the pan and place it in the slow cooker. Add the mushrooms to the pan and cook for 4-5 minutes or until tender.
Add the garlic to the pan and cook for 30 seconds.
Place the mushroom mixture, onion, bell peppers, Italian seasoning and marinara sauce in the slow cooker.
Gently toss all the ingredients together.
Cover and cook on LOW for 6-8 hours or HIGH for 3-4 hours.
Remove the cover and stir in the olives. Sprinkle the parsley over the top and serve.
The chicken will release liquid as it cooks, which will thin out the sauce. If you feel like your sauce is too thin, you can thicken it up by uncovering the slow cooker and running it for 20 minutes on high heat.