Heat a non stick pan with with the oil, when hot, add the chicken, working in batches if needed, and brown each side. When browned, place in your slow cooker. (These 3 chicken breasts were about 800 grams.)
Once all the chicken is done, using the same pan, add the onion, cook, stirring often until tender. Remove from heat and add the onion on top of the chicken in the slow cooker.
Now add the mushrooms, garlic, and parsley to the slow cooker. (I used cilantro in place of the parsley, worked perfectly, and I did use my 3½ quart slow cooker for this.)
Pour in the chicken broth, season with salt and pepper as desired, tamp everything down level, no need to make sure in broth as it will make liquid shortly. Cover and set the cooker to Low setting for 4 hours.
When 4 hours have passed, leave the slow cooker on Low setting and remove the chicken from the cooker with a slotted spoon, leaving as much liquid as you can and the mushrooms in the cooker.
Whisk in the cornstarch into the cooker until smooth.
Return the chicken to the cooker and cook until the sauce has thickened, about 20-30 minutes.
Server over pasta, rice, or mashed potatoes, and the gravy is excellent. Enjoy.