Heat the oven to 375°F. Line a small baking sheet or loaf pan with parchment paper, or lightly grease it with a little of the measured olive oil.
Put the sweet potato chunks in a saucepan and cover with water. Bring to a boil, then simmer until very tender, about 12 to 15 minutes. Drain well and mash until smooth. Measure out ½ cup for the meatloaf and let it cool until just warm.
While the sweet potato cooks, warm the olive oil in a small skillet over medium heat. Add the minced shallot and cook for 3 to 4 minutes, stirring, until softened. Let it cool.
In a large bowl, combine the ground turkey, grated apple, egg, ½ cup mashed sweet potato, cooked shallot, sage, Dijon mustard, salt, and pepper. Mix gently with your hands or a fork just until combined.
Shape the mixture into a compact loaf on the prepared baking sheet, or press it into the loaf pan. Bake until the center reaches 165°F, about 45 to 55 minutes.
Let the meatloaf rest for 10 minutes before slicing. This helps the slices hold together and keeps the juices in the loaf.