Heat the oven to 375°F. Line a small baking sheet or loaf pan with parchment paper.
Peel the sweet potato and cut it into small cubes. Place the cubes in a saucepan, cover with water, and bring to a boil. Cook until fork-tender, about 10 to 12 minutes. Drain well, mash until smooth, and let it cool slightly.
While the sweet potato cooks, warm the olive oil in a small skillet over medium heat. Add the diced onion and cook for 4 to 5 minutes, stirring occasionally, until softened. Let it cool.
In a large bowl, combine the ground turkey, mashed sweet potato, egg, oats, pecans, cooked onion, maple syrup, dried sage, salt, and black pepper. Mix gently until just combined.
Shape the mixture into a compact loaf on the prepared baking sheet, or press it into the prepared 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.