Preheat the Traeger to 375°F. Cut the top off the garlic head to expose the cloves, drizzle with olive oil, and wrap it in foil. Roast the garlic on the grill until soft, about 35 to 45 minutes. Let it cool, then squeeze the cloves from their skins and mash them.

While the garlic roasts, combine the ground beef, egg, breadcrumbs, whole milk, onion, Worcestershire sauce, kosher salt, and black pepper in a bowl. Mix gently just until combined.

Shape the mixture into a compact loaf on a rimmed baking sheet or in a grill-safe pan. Lay the bacon slices across the top, tucking the ends under the loaf. Brush the bacon with half of the ketchup.

Put the meatloaf on the Traeger and smoke at 225°F until the center reaches 150°F, about 1 1/2 to 2 hours. Raise the grill temperature to 375°F, brush on the remaining ketchup, and cook until the center reaches 160°F and the bacon is browned, about 20 to 30 minutes more. Rest the meatloaf for 10 minutes before slicing.

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

Mash the potatoes with the butter, warm milk, roasted garlic, and 1/2 teaspoon kosher salt. Add a little more warm milk if needed for a softer texture. Serve the mashed potatoes with slices of bacon-wrapped meatloaf.