How to Make It
Preheat oven to 350°F. Place a pan of water on bottom rack.
Heat oil in a large skillet over medium-high. Add onion; sauté 3 minutes. Add mushrooms; sauté 3 minutes. Add Worcestershire, thyme, garlic, 1/2 teaspoon salt, and 1/4 teaspoon pepper; sauté 1 minute.
Gently combine beef, panko, eggs, mushroom mixture, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper in a large bowl. For easy cleanup, line an 8 1/2- x 4 1/2-inch loaf pan with foil; let foil overhang edges. Add meat mixture; lightly press into edges. Bake at 350°F on middle rack until loaf center is 160°F, about 1 hour.
Combine ketchup, sugar, and vinegar; brush over loaf top. Increase heat to broil. Broil loaf 10 minutes or until glaze bubbles. Remove from oven; cool 10 minutes. Lift or remove loaf from pan. Cut into 6 slices.