Preheat oven or grill to 350°F. Line a baking sheet with parchment paper or foil for easy cleanup.
In a small bowl, combine bread crumbs and milk. Stir to combine.
Melt the butter in a large sauté pan over medium-high heat. Cook the onions until they are fragrant and slightly brown while stirring occasionally, about 3-5 minutes. Reduce the heat to low and add the garlic. Cook for 2-3 additional minutes. Remove the pan from heat and set it aside to cool.
In a large bowl, combine the ground beef, egg, soaked bread crumbs with the milk, Worcestershire sauce, thyme, 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 ketchup with mustard and 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).