Mix ground sirloin, panko breadcrumbs, egg, milk,1/2 package onion soup mix, Worchestershire sauce, and black pepper together in a large bowl. Shape into 5 patties.
Step 2
Heat a skillet over medium heat. Cook patties in hot skillet until browned, 3-5 minutes per side.
Step 3
Melt butter in a separate skillet over medium-high heat. Saute mushrooms and onion in melted butter until tender, 5 to 7 minutes. Stir flour and remaining dry onion soup mix into the mushroom mixture; cook and stir until flour is integrated fully, about 1 minute. Stream beef stock and water over the mushroom mixture while stirring continually; bring to a simmer, reduce heat to medium, and cook, stirring frequently, until the liquid thickens, about 5 minutes. Season with salt and pepper.
Step 4
Lie the browned steaks into the gravy; simmer until steaks are firm and gray in the center, about 30 minutes. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).