In a large bowl combine the ground beef with minced onion and mustard. Season with salt and pepper and mix until combined. Divide the meat into 4 oval patties, 1/2 inch in thickness.
In a skillet, heat the olive oil over medium-high heat and cook the patties for 5 minutes each side until browned. Remove from the skillet and transfer to a plate. Set aside.
In the same skillet, melt the butter and cook the onions for 5 minutes until translucent. Add the flour and cook for 30 seconds while stirring. Gradually add the beef broth while whisking. Bring the mixture to a boil and simmer for 2 minutes until the sauce thickens. Season the gravy with salt and pepper.
Return the hamburger patties to the skillet and spoon the gravy over the burgers. Cook for 5 more minutes then remove the skillet from the heat.
Garnish the Hamburger steaks with parsley and serve over mashed potatoes. Enjoy!