In a large mixing bowl, combine ground beef, finely chopped onion, minced garlic, Worcestershire sauce, Dijon mustard, egg, breadcrumbs, salt, and black pepper. Gently mix until just combined, being careful not to overwork the meat. Shape into four oval patties, about ½ inch thick.

Heat olive oil in a large skillet over medium-high heat. Once hot, add the steak patties and sear for about 4-5 minutes per side until they develop a deep golden-brown crust. Remove from the pan and set aside.

In the same skillet, melt butter over medium heat. Add sliced mushrooms and chopped onions, sautéing for about 5-7 minutes until softened and caramelized. Pour in the beef broth, Worcestershire sauce, and Dijon mustard, stirring to deglaze the pan and incorporate the browned bits.

In a small bowl, mix cornstarch and water to create a slurry. Slowly add this to the pan while stirring continuously to thicken the gravy. Let simmer for another 5 minutes, then stir in the heavy cream if a richer texture is desired.

Return the Salisbury steak patties to the skillet, spooning the hot mushroom gravy over them. Simmer for a few more minutes to allow the flavors to meld. Serve immediately over mashed potatoes, rice, or buttered egg noodles.