Heat olive oil in a large skillet. Add onions, red pepper, garlic, mushrooms and zucchini. Cook for 5-10 minutes, until onions are translucent and mushrooms are browned. Season to taste with salt and pepper
Pour cooked veggies into a large crock pot. Add remaining ingredients and stir to combine. Cover and cook on low for 6-8 hours or on high for 4 hours.
Before serving, remove chicken and shred or cut into cubes. Stir back into the sauce. Serve over pasta, if desired.