Heat the oven to 400°F. Lightly grease 8 cups of a standard muffin tin.
In a large bowl, combine the ground turkey, breadcrumbs, egg, grated onion, 1 tablespoon of the ketchup, Worcestershire sauce, Dijon mustard, salt, and black pepper. Mix gently just until combined.
Divide the turkey mixture among the 8 prepared muffin cups. Shape it into even mounds, then brush the tops with the remaining 1 tablespoon of ketchup. Bake for 20 to 25 minutes, until the centers reach 165°F.
While the meatloaf muffins bake, place the potatoes in a saucepan and cover them with cold water. Bring to a boil, then simmer until the potatoes are very tender, about 15 minutes. Drain well.
Return the potatoes to the saucepan. Add the butter, warm milk, grated garlic, parsley, and dried thyme. Mash until smooth and fluffy. If using a piping bag, let the potatoes cool slightly so the frosting holds its shape.
Let the muffins rest for 5 minutes after baking. Spoon or pipe the garlic-herb mashed potatoes over each one and serve warm.