If you haven’t already, cook the quinoa or other grain according to package directions. Drain in a strainer and rinse. Meanwhile, peel the sweet potato and dice it into 1/2-inch or smaller pieces. Cook it in boiling water to cover until tender, about 15 minutes; drain well.
Toss together the quinoa, potato, bell pepper, and onion; sprinkle with salt and pepper. Whisk the oil and vinegar together and toss the salad with about half of this mixture; add all or some of the rest to taste. Taste and adjust the seasoning, garnish with the chives (or parsley) and serve.