Heat the oven to 375°F. Line a small rimmed baking sheet with parchment paper.

In a large bowl, combine the breadcrumbs and 1/4 cup milk. Let them sit for 2 minutes so the crumbs soften. Add the egg, chopped onion, Worcestershire sauce, kosher salt, black pepper, garlic powder, and 2 tablespoons of the ketchup. Mix to combine.

Add the ground turkey and gently mix with your hands or a fork until everything is evenly distributed. Avoid packing the mixture tightly. Shape it into a compact loaf on the prepared baking sheet.

In a small bowl, stir together the remaining ketchup, brown sugar, and yellow mustard. Spoon the glaze over the top and sides of the loaf.

Bake for 45 to 55 minutes, until the center reaches 165°F on an instant-read thermometer. Let the meatloaf rest for 10 minutes before slicing.

While the meatloaf bakes, place the potatoes in a large pot and cover them with cold water. Bring the water to a boil, then reduce the heat and simmer until the potatoes are very tender, about 15 to 20 minutes. Drain well.

Return the potatoes to the pot. Add the butter, warm milk, and 1/2 teaspoon salt. Mash until smooth, adding a little more warm milk if needed for a softer texture.

Slice the rested meatloaf and serve it with the mashed potatoes.