In a mixing bowl, combine the ground beef, bread crumbs, egg, Worcestershire sauce, garlic powder, onion powder, salt, and black pepper. Mix gently with your hands until just combined; overmixing can lead to tough steaks.

Form the mixture into 4 oval patties, about ½ inch thick. Make a slight indentation in the center of each patty to prevent them from puffing up during cooking.

Heat a skillet over medium-high heat and add a little oil. Once hot, add the patties and cook for about 3-4 minutes on each side until browned. This step adds flavor through the Maillard reaction.

Remove the patties from the skillet and set aside. Drain excess fat from the skillet, leaving about 1 tablespoon.

In the same skillet, add the sliced onion and mushrooms. Sauté for about 5 minutes until they are softened and lightly browned. This enhances their flavor.

Pour in the beef broth and soy sauce, scraping the bottom of the skillet to deglaze and incorporate any browned bits. Bring to a simmer.

Transfer the browned patties to the slow cooker. Pour the mushroom and onion gravy over the patties.

Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the patties are cooked through and tender.

About 30 minutes before serving, mix the cornstarch with the water in a small bowl to create a slurry. Stir this into the slow cooker to thicken the gravy. Cover and continue cooking.

Once cooked, serve the Salisbury steaks with the gravy over mashed potatoes or rice for a complete meal.