In a large bowl, combine ground chicken, egg, diced yellow onion, breadcrumbs, garlic, salt & pepper and mix well. Using a large tablespoon or cookie scoop, scoop & roll into meatballs. Chill in freezer for 10-15 minutes.
In a large pan over medium heat add olive oil and place meatballs in a single layer, making sure they don't touch (this may need to be done in multiple batches).
Cook for 4-5 minutes per side until the meatballs are browned and cooked through, then remove from the pan
Add chicken broth, dijon mustard, maple syrup and heavy cream into the pan, scraping off any brown bits from the bottom and mixing to evenly to combine. Bring to a simmer.
Whisk together corn starch and cold water, then add to the sauce and mix again until well combined. Add meatballs back in and simmer for another 4-6 minutes
Top with fresh parsley & serve over rice!