Begin by placing the sliced mushrooms and onions at the bottom of the crockpot.
In a medium-sized mixing bowl, combine the ground beef, bread crumbs, ketchup, Worcestershire sauce, mustard powder, egg, salt, and pepper. Mix thoroughly with your hands until all ingredients are well incorporated. Form the mixture into four oval patties.
Heat the butter in a skillet over medium-high heat. Brown the patties on each side for about 3 minutes, ensuring they are lightly seared but not fully cooked through.
Arrange the browned patties over the mushrooms and onions in the crockpot.
In a separate bowl, whisk together the beef broth, ketchup, and Worcestershire sauce. Pour the mixture over the patties in the crockpot.
Cover and cook on high for 3 hours or low for 5 hours, allowing the flavors to meld and the patties to become tender.
Once the cooking time is complete, remove the patties from the crockpot and set aside. In a small bowl, mix the cornstarch and water to create a slurry. Stir this mixture into the broth, allowing it to thicken over a few minutes.
Return the beef patties to the crockpot and gently toss them in the thickened sauce to coat.
Serve the Salisbury steak over mashed potatoes, garnished with freshly chopped parsley for added flavor and visual appeal.