1. Toast pistachios in 12-inch nonstick skillet over medium heat until lightly browned, stirring frequently, about 3 minutes. Transfer to small bowl; set aside.

2. Heat butter in now-empty skillet over medium-high heat; when foaming subsides, swirl to coat skillet. Add onion and celery root in even layer; cook without stirring until browned, about 3 minutes. Stir and continue to cook, stirring occasionally, until all sides are browned, about 3 minutes longer. Add broth, sugar, vinegar, salt, and pepper; cover skillet, reduce heat to medium-low, and simmer until vegetables are just tender, about 10 minutes.

3. Uncover, increase heat to high, add grapes, and cook, stirring frequently, until liquid in skillet reduces to glaze, about 1 minute. Transfer to serving dish and sprinkle with pistachios; serve immediately.

Serves 4 as a side dish