In a small food processor pulse pecans until finely ground. Add 2 tablespoons sugar and yolk and pulse until combined. Halve and pit nectarines and arrange, cut sides up, on a small baking sheet. Divide pecan mixture among nectarine halves, mounding it in center, and garnish each mound with a pecan half. Sprinkle nectarines with remaining 1/2 tablespoon sugar.
Bake stuffed nectarines in middle of oven until pecan mixture is golden, about 10 minutes.
