Preheat the oven to 350°F and lightly grease a loaf pan.
In a large bowl, combine the ground beef, breadcrumbs, egg, 1/2 cup milk, chopped onion, 1 teaspoon salt, and black pepper. Mix until evenly blended but do not overwork.
Press the mixture into the prepared loaf pan and smooth the top.
In a small bowl, stir together the ketchup, brown sugar, and yellow mustard until smooth. Spread half of this sauce over the meatloaf.
Bake for 45 minutes. Remove the pan, spread the remaining sauce on top, and return to the oven for 15 more minutes.
While the meatloaf finishes baking, place the potatoes in a pot of salted water and boil until fork-tender, about 15 minutes.
Drain the potatoes, then return them to the pot. Add the butter, 1/2 cup milk, and 1/2 teaspoon salt. Mash until creamy.
Let the meatloaf rest 5 minutes before slicing. Serve slices with a scoop of mashed potatoes and any extra sauce from the pan.