Preheat oven or grill to 350°F. Line a baking sheet with parchment paper or foil for easy cleanup.

In a small bowl, combine 1 cup panko bread crumbs and 1 cup milk. Stir to combine.

Melt the 1 tablespoon butter in a large sauté pan over medium-high heat. Cook the diced yellow onion until they are fragrant and slightly brown while stirring occasionally, about 3-5 minutes. Reduce the heat to low and add 2 cloves garlic. Cook for 2-3 additional minutes. Remove the pan from heat and set it aside to cool.

In a large bowl, combine the 2 pounds ground beef, 1 large egg, soaked bread crumbs with the milk, 1 tablespoon Worcestershire sauce, 1/2 teaspoon dried thyme, 1 teaspoon kosher salt, and the cooked onions and garlic. Use your hands to combine the mixture, but try not to overwork the meat. Shape the mixture into a loaf on the parchment-lined baking sheet. Use a sharp knife to cut a crisscross pattern on top.

Bake uncovered in the preheated oven 40 minutes. While the meatloaf is cooking, combine 1/2 cup ketchup with 1 tablespoon spicy brown mustard and 1 tablespoon brown sugar in a bowl. Set aside.

After 40 minutes of cooking, spread the sauce mixture over the top of the meatloaf and continue baking for about 30-35 minutes to an internal temperature of 160°F (70°C).