Preheat the oven to 350°F.
In a large bowl mix together the ground beef, egg, breadcrumbs, ketchup, dijon mustard, Worcestershire sauce, black pepper, salt, and crushed beef bouillon. You can mix by hand, be careful to not over mix.
Divide the mixture into 8 even portions and form the portions into flat oval shapes.
Heat an oven-safe 12-inch skillet (I like to use cast iron) with deep sides over medium heat.
Add the steaks directly into the pan, no need to add any extra oil.
Brown salisbury steaks on both sides, about 2-3 minutes per side, then place the pan into the oven for 10-15 minutes until an internal temperature reaches 165°F.
Take the steaks out of the pan and place them on a plate and cover them with foil, set aside.
Wipe out any excess grease from the pan.
Over medium heat, add the butter and let it melt.
Add the onion and mushrooms and cook, stirring occasionally, for about 5 minutes until the onions are softened.
Add the garlic and stir it in until fragrant, 30 seconds.
Add the flour and stir it in, coating the veggies, cook stirring constantly for 2 minutes.
Slowly whisk in the beef stock, making sure there are no lumps.
Add the Worcestershire sauce, dijon mustard, and black pepper, whisk it in. Taste and add additional sauce if needed.
Let the gravy simmer for 3 minutes, stirring occasionally.
Add the steaks back to the pan and cover in the sauce, let simmer for 5 more minutes or until the steaks are heated through.
Garnish with optional parsley and serve.