- 1-1/2 pounds 93 percent lean ground beef
- 2 tablespoons plus 2 teaspoons water
- 1/2 teaspoon baking soda
- Salt and pepper
- 2-1/2 pounds russet potatoes, peeled and cut into 1‐inch chunks
- 2 tablespoons unsalted butter, melted
- 1/2 cup 2 percent low‐fat milk
- 1 large egg yolk
- 8 scallions, green parts only, sliced thin
- 2 teaspoons vegetable oil
- 1 onion, chopped
- 4 ounces white mushrooms, trimmed and chopped
- 1 tablespoon tomato paste
- 2 garlic cloves, minced
- 2 tablespoons Madeira or ruby port
- 2 tablespoons all‐purpose flour
- 1-1/4 cups beef broth
- 2 teaspoons Worcestershire sauce
- 1 bay leaf
- 2 sprigs fresh thyme
- 2 carrots, peeled and chopped
- 2 teaspoons cornstarch
Toss ground beef with 2 tablespoons water, baking soda, 1 teaspoon salt, and 1/4 teaspoon pepper in bowl until well combined; let sit for 20 minutes.
Meanwhile, place potatoes and 1 tablespoon salt in large saucepan; add water to cover. Bring to boil, then reduce to simmer and cook until potatoes are tender, 8 to 10 minutes. Drain potatoes; return to saucepan over low heat, stirring constantly, to evaporate any remaining moisture, about 1 minute. Off heat, mash potatoes smooth with potato masher. Stir in butter. Whisk milk and egg yolk together in bowl, then stir into potatoes. Stir in scallions; season with salt and pepper to taste. Cover; set aside.
Heat oil in broiler-safe 10-inch skillet over medium heat until shimmering. Add onion and mushrooms and cook until vegetables begin to soften, 4 to 6 minutes. Stir in tomato paste and garlic and cook until bottom of skillet is dark brown, about 2 minutes. Stir in Madeira, scraping up any browned bits, and cook until evaporated, about 1 minute. Stir in flour and cook for 1 minute. Stir in broth, Worcestershire, bay leaf, thyme, and carrots; bring to boil, scraping up any browned bits.
Reduce heat to medium-low. Pinch beef into 2-inch pieces and lay on top of mixture in skillet. Cover and cook until beef is cooked through, 10 to 12 minutes, stirring and breaking up meat chunks with two forks halfway through cooking time. Combine cornstarch and remaining 2 teaspoons water in bowl, then stir into skillet. Continue to simmer, stirring constantly, until filling is slightly thickened, about 30 seconds. Remove thyme and bay leaf. Season with pepper to taste.
Adjust oven rack 6 inches from broiler element and heat broiler. Place mashed potatoes in large zipper-lock bag; snip off corner to create 1-inch opening. Pipe potatoes evenly over filling to cover entire surface. Smooth potato with back of spoon, then use tines of fork to make ridges over surface. Place skillet on rimmed baking sheet; broil until potatoes are golden and sauce is bubbling, 10 to 15 minutes. Let cool slightly; serve.