Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper.
In a large bowl, combine the ground turkey, egg, breadcrumbs, grated onion, garlic powder, salt, black pepper, 1 tablespoon of the Dijon mustard, and 1 tablespoon of the maple syrup. Mix gently until just combined.
Shape the mixture into a compact loaf on the prepared baking sheet. Stir the remaining Dijon mustard and maple syrup together in a small bowl, then brush the mixture over the top and sides of the loaf.
Bake until the center reaches 165°F on an instant-read thermometer, about 45 to 55 minutes. Let the meatloaf rest for 10 minutes before slicing.
While the meatloaf bakes, place the sweet potato chunks in a saucepan and cover with water. Bring to a boil, then reduce the heat and simmer until the potatoes are tender when pierced with a fork, about 15 to 20 minutes.
Drain the sweet potatoes and return them to the saucepan. Add the butter and milk, then mash until smooth. Serve the mash alongside slices of meatloaf.