Step 1Heat the oven to 325 degrees. Line a rimmed baking sheet with foil and place a 9-by-5-by-4-inch loaf pan on top.
Step 2Tear the bread into small pieces over a large bowl. Pour the milk evenly over the bread and let stand for 5 minutes to soften. Stir the bread until it forms a mostly smooth paste. Add the onions, 6 tablespoons ketchup, the seasoned salt, steak sauce, Worcestershire sauce, thyme, garlic and eggs and stir until evenly combined.
Step 3Working over the bowl, crumble the beef into small pieces, letting them fall in the sauce. Using your hands, gently toss the meat with the other ingredients until everything is evenly combined, making sure not to knead the beef too much. Immediately transfer the beef to the prepared loaf pan and pat it gently to smooth the top.
Step 4In a small bowl, combine the remaining 1/2 cup ketchup, the brown sugar and mustard and stir until smooth. Pour the sauce over the top of the meatloaf and spread it into an even layer. Place the meatloaf on the baking sheet in the oven and bake until an instant-read thermometer inserted in the center reads 155 degrees, about 1 hour and 10 minutes.
Step 5Remove the meatloaf from the oven and let rest for 10 minutes before slicing and serving from the pan.