1. Combine farro, water and ¾ teaspoon salt in a large heavy saucepan and bring to a boil. Stir and reduce the heat to a simmer; cook, uncovered, until the farro is tender, 35 to 40 minutes.
2. Meanwhile, heat 2 tablespoons oil in a medium skillet over medium heat. Add onion and garlic and cook, stirring, until translucent, 4 to 6 minutes. Remove from the heat.
3. Combine pistachios, the remaining ½ teaspoon oil and ¼ teaspoon pepper in a large bowl; toss to combine.
4. Drain the farro and add to the bowl along with the onion mixture and parsley. Season with the remaining ¼ teaspoon salt and pepper. Toss to combine.

Tip: Toast pistachios in a small dry skillet over medium-low heat, stirring constantly, until fragrant and lightly browned, 4 to 6 minutes. To make ahead, prepare up to 2 hours ahead; hold at room temperature and reheat over low until warm.