1 1/2 cups quinoa (any color) rinsed well
1 medium red onion, sliced ¼” thick
1/2 cup apple cider vinegar
3 Tbsp. sugar
2 large ripe firm peaches, cut into ½” pieces
1/2 bunch arugula, thick stems trimmed, leaves torn (about 2 cups)
2 cups small cherry tomatoes (about 1 pint), halved
1/4 cup olive oil
1/2 cup ½” pieces chives, divided
4 tsp. kosher salt plus more
Freshly ground black pepper
Bring quinoa and 4 cups water to a boil in a medium saucepan. Season with salt. Cover, reduce heat, and simmer until quinoa is tender, 8–10 minutes. Drain, return quinoa to pan, and cover. Remove from heat and let sit 15 minutes. Fluff with a fork and spread out on a rimmed baking sheet; let cool.
Meanwhile, place onion in a small bowl. Bring vinegar, sugar, and 4 tsp. salt to a boil in a small saucepan, stirring to dissolve sugar and salt. Pour over onion and let stand 20 minutes. Drain, reserving pickling liquid.
Toss pickled onion, peaches, arugula, tomatoes, oil, ¼ cup chives, and 3 Tbsp. reserved pickling liquid in a large bowl; season with salt, pepper, and more pickling liquid, if desired. Fold in quinoa.
Serve salad topped with remaining ¼ cup chives.