Lower heat to medium, add onions and a pinch of salt and cook until slightly softened (approx. 3-4 min). Add garlic and cook an additional 60 seconds.
Deglaze pan with ¼ cup beef stock. Use a wooden spoon to scrape any brown bits on the bottom of the pan. Add the remaining beef stock, ketchup, mustard, fish sauce, and stir to combine. Bring sauce to a simmer and cook until reduced in volume by half (approximately 6-8 min).
In a small bowl, stir together arrowroot starch and water to make a slurry. Add to gravy and stir to combine. Taste gravy for seasoning and adjust salt and pepper as required. Return browned meatballs to pan along with chopped parsley and toss everything to coat.
Serve Salisbury Meatballs and Onion Gravy on a bed of mashed potatoes or puréed cauliflower and a side of steamed green beans or vegetables of choice.