In a large bowl, combine the ground beef, diced onion, breadcrumbs, minced garlic, egg, ketchup, mustard, Worcestershire sauce, salt, pepper, and parsley. Mix gently until just combined—avoid overmixing to keep the meatballs tender. Roll the mixture into 1½ inch meatballs. For best results, refrigerate the meatballs for 15 minutes to help them hold their shape during cooking.

Heat the olive oil in a large nonstick skillet over medium heat. Add 5-6 meatballs at a time, browning them for about 2 minutes on each side. Make sure to leave space between the meatballs for easy flipping. Once browned, remove them from the skillet and set aside. Repeat with the remaining meatballs.

Lower the heat to medium and add the butter to the skillet. Add the sliced onions and cook, stirring often, for about 15 minutes until they soften and caramelize slightly.

Pour in the chicken broth, beef broth, bouillon cube, onion powder, garlic powder, and Worcestershire sauce. Stir well to combine, scraping any brown bits from the bottom of the pan. If desired, add 2-3 drops of Kitchen Bouquet for a richer color.

In a small container with a lid, combine the corn starch and cold water. Shake well to mix. Bring the gravy to a boil, then slowly whisk in the cornstarch mixture until the gravy reaches your desired thickness. Reduce the heat to medium-low and continue whisking to ensure a smooth blend.

Return the browned meatballs to the skillet, spooning the gravy over them. Allow them to heat through for about 10 minutes, ensuring they are cooked all the way. Garnish with fresh parsley before serving.