Peel the onions, cut them into quarters, and place them in the bottom of the slow cooker.
Place the whole chicken on top of the onions. (There is no need to add any liquid as the chicken and onions will release juices throughout the cooking process.)
Pour some olive oil over top of the chicken, and use a pastry brush to spread it over the the entire bird. (You can also choose to spread butter over top instead, and place slivers of butter under the skin.)
Sprinkle salt, pepper, and other herbs of choice over the chicken.
Place the top on the slow cooker, and turn it to high heat for 5 hours.
When the five hours is up, you can either choose to serve the chicken as is, or place it under the broiler of your oven for a few minutes to crisp the skin.
You can serve the soft onions alongside the chicken, or use an immersion blender to blend them into the juices of the chicken and serve as a sauce to accompany it. (It can also be made into gravy by thickening the sauce with a bit of wheat or tapioca flour, depending on your diet.)