In a large mixing bowl, combine ground beef with egg, breadcrumbs, salt, black pepper, and grated shallot. Mix until just combined. Divide the mixture into four equal portions and shape into balls.

Heat olive oil and unsalted butter in a skillet over medium-high heat. Press the beef balls down into ¾-inch thick patties and cook for about 5 minutes on each side until browned.

Remove the patties from the skillet and set aside. Lower heat and add garlic and shallots, sautéing for 1 minute until fragrant.

Stir in yellow mustard, salt, pepper, and flour to absorb the pan drippings. Deglaze with whiskey and simmer for about 3 minutes until it thickens.

Pour in beef broth and bring to a boil. Lower heat and stir in cream until fully incorporated.

Return hamburger steaks to the skillet, spooning sauce over each patty. Let simmer for about 5 minutes.

Remove from heat and garnish with chopped chives. Serve immediately with extra sauce.