Makes 5 patties
Patties:
1 pound ground sirloin
½ cup panko bread crumbs
2 tablespoons milk
½ (1 ounce) package dry onion soup mix
1 teaspoon Worcestershire sauce
¼ teaspoon ground black pepper
Gravy:
3 tablespoons butter
2 cups fresh mushrooms, sliced
1 sweet onion, sliced
3 tablespoons all-purpose flour
½ (1 ounce) package dry onion soup mix
1 ½ cups beef stock
1 cup water
salt and ground black pepper to taste
Mix ground sirloin, panko bread crumbs, milk, and 1/2 packet onion soup mix, Worcestershire sauce, and black pepper together in a large bowl; shape into 5 patties.
Heat a skillet over medium heat. Cook patties in hot skillet until browned, 3 to 5 minutes per side.
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.
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).