Step 1

Mix ground sirloin, panko bread crumbs, egg, milk, and 1/2 packet onion soup mix, Worcestershire 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 to 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).