Prepare the Salisbury Steaks: In a mixing bowl, combine the ground beef, breadcrumbs, milk, egg, Worcestershire sauce, thyme, salt, and black pepper. Mix gently until just combined—avoid overmixing to keep the steaks tender. Form the mixture into 4–6 oval-shaped patties, about ½-inch thick. Heat olive oil in a large skillet over medium-high heat. Add the patties and cook for 3–4 minutes per side, until browned on the outside. Remove and set aside.

Make the Garlic Mashed Potatoes: Place the diced potatoes in a large pot and cover with cold water. Add a pinch of salt and bring to a boil. Cook for 15–20 minutes, until the potatoes are fork-tender. Drain and return them to the pot. Add the milk, butter, minced garlic, salt, and black pepper. Mash until smooth and creamy. Keep warm while preparing the gravy.

Prepare the Mushroom & Onion Gravy: In the same skillet used for the steaks, melt the butter over medium heat. Add the sliced onion and cook for 5–7 minutes, until softened and caramelized. Stir in the mushrooms and cook for 5–6 minutes, until golden brown and tender. Sprinkle the flour over the onions and mushrooms, stirring to coat. Cook for 1 minute to remove the raw flour taste. Gradually whisk in the beef broth, Worcestershire sauce, and soy sauce (if using). Bring to a simmer, stirring constantly, until the gravy thickens slightly. Season with salt and black pepper to taste.

Combine and Simmer: Return the Salisbury steaks to the skillet with the gravy. Spoon the gravy over the steaks and simmer for 5–10 minutes, until the steaks are cooked through (internal temperature should reach 160°F/71°C) and the flavors meld together.

Serve and Enjoy: Divide the creamy garlic mashed potatoes among plates or bowls. Top each serving with a Salisbury steak and spoon the mushroom and onion gravy generously over the top. Garnish with freshly chopped parsley or thyme leaves for added brightness. Serve warm and enjoy every bite of this hearty, comforting meal!