In a medium sauté pan, add the olive oil, butter and onions. Cook over medium-low heat stirring a few times until the onions are soft and golden brown, about 20-25 minutes.
While the onions are cooking, make the meatballs. Preheat the oven to 375°F.
Combine the panko crumbs and milk together and set aside for 5 minutes.
In a large bowl add the ground beef, tomato paste, egg, beef bouillon, Worcestershire sauce, dry mustard, parsley, salt, and black pepper.
Add the wet panko crumbs to the beef and mix just until everything is evenly incorporated.
Using a medium scoop (#40) portion out the meatballs and place on a large parchment lined sheet pan. Then roll the meatballs so they are smooth.
Bake the meatballs for 25 minutes.
While the meatballs are baking, make the gravy. In a large deep skillet, bring the beef broth to a boil over medium-high heat.
Whisk together the cornstarch and water.
Once the broth is boiling, whisk in the cornstarch slurry, Worcestershire sauce and beef bouillon. Reduce heat to medium-low.
Keep whisking until the gravy has thickened. Season the gravy with salt and pepper.
Add the cooked onions and meatballs to the gravy.
Serve over mashed potatoes, rice or noodles.