Make dressing: Combine all ingredients in a jar, screw on lid, and shake vigorously until well combined (or use an immersion blender). Shake again just before using.
Make salad: Cook quinoa according to package directions. Position a rack in top third of oven and preheat broiler to high. Spread cooked quinoa on a rimmed baking sheet. Toast quinoa under broiler, stirring occasionally and checking frequently, until it begins to crisp and turn golden brown around edges, 10 to 12 minutes. Let cool. Place lettuce in a large salad bowl. Add quinoa, oranges, avocado, scallions, and dressing. Toss well to combine and serve immediately.