Place breadcrumbs in a large bowl and grate the onion directly over them using a box grater. Mix with your fingers and let sit for 1-2 minutes so the breadcrumbs absorb the onion juices. Add remaining patty ingredients and mix with your hands until just combined, then continue mixing for another 2 minutes until the mixture becomes slightly pasty.

Divide the mixture into 5 equal portions and pat them very firmly into oval patties about 3/4 inch thick. Press them together well to prevent crumbling during cooking.

Heat oil in a 12-inch skillet over high heat for about 1 minute until it shimmers. Add patties and sear for 1 minute per side until browned but still raw inside. Transfer to a plate.

Reduce heat to medium-high. Add chopped onion and garlic to the same skillet and cook for 2 minutes until softened. Add mushrooms and cook for 2-3 minutes until golden brown.

Turn heat to medium and add butter. Once melted, sprinkle in flour and stir constantly for 30 seconds. Gradually add beef broth in three additions, stirring well after each. Whisk in water, mustard, and Worcestershire sauce.

Return patties to the skillet along with any accumulated juices. Simmer for 5-7 minutes, occasionally stirring the gravy around the patties, until the gravy coats the back of a spoon and patties reach 160°F internally. If gravy thickens too quickly, add more water. Taste and adjust salt and pepper as needed.

Serve Salisbury Steak over mashed potatoes, topped with plenty of mushroom gravy. Garnish with fresh parsley if desired.