Season the chicken with the salt and pepper and brown it in the oil over medium-high heat, 4 minutes per side.
Transfer the chicken to a 6-quart slow cooker and cook the onion and mushrooms in the skillet until browned, 6 minutes.
Add the garlic for 30 seconds, then transfer the vegetables to the cooker.
Pour in the tomatoes and wine and sprinkle in the oregano and basil.
Cover and cook on low for 3 1/2 hours, then stir both bell peppers into the sauce around the chicken.
Cover again and cook until the peppers are tender and the chicken reaches 175°F, 2 to 2 1/2 hours more.
Skim the sauce, finish with the parsley and serve one thigh per person with plenty of mushrooms and peppers.