Place chicken in bottom of slow cooker, in a single layer. Sprinkle with garlic and onions. Combine all remaining ingredients - EXCEPT cornstarch - and pour over top. Place lid on slow cooker and cook for 7 hours on low.
In the last 30 minutes of cooking, remove ½ cup of liquid from slow cooker and whisk in corn starch. Return liquid to the slow cooker and stir to combine. Replace lid and continue to cook for remainder of cook time.
Serve with a drizzle of "pan gravy" over top of chicken and garnish with lemon slice and/or parsley if desired.