How to Make It
On a shallow platter, combine 2 tablespoons of the olive oil with 2 tablespoons of the vinegar and season generously with salt and pepper. Add the onion slices and turn to coat, keeping the slices intact; let stand at room temperature for 30 minutes.
Light a grill or preheat the broiler. Grill the onions over a medium-hot fire or broil for about 8 minutes, until softened and charred in spots. Return the onions to the platter and let cool. Coarsely chop the onions and transfer them to a large bowl.
In a large pot of boiling salted water, cook the fregola until al dente. Drain well, shaking off any excess water. Return the fregola to the pot, add 1/4 cup of the oil and toss to coat. Spread the fregola out on a large rimmed baking sheet to cool.
Meanwhile, in a medium skillet, heat 1 tablespoon of the olive oil. Add the pine nuts and cook over moderate heat, stirring constantly, until golden, about 4 minutes. Drain on paper towels and let cool. Add the remaining 1 tablespoon of olive oil to the skillet along with the carrots and celery and cook over moderate heat until softened, about 4 minutes. Transfer to a plate and let cool.
Add the fregola to the bowl with the grilled onions. Add the carrots and celery, pine nuts, cilantro, parsley and the remaining 1 tablespoon of vinegar. Season with salt and pepper and serve.