First, take out a small bowl and toss in the onions. Add in some warm water, a dash of salt, and the lime juice. Let marinate for 10 minutes. While the onions are marinating; let's make the dressing. Take out a mason jar (or something with a lid) and combine all the ingredients for the dressing. Shake well. Set aside. Next, take out a large bowl, and add the arugula, the sliced mangoes, the sliced avocado, and the red onions. Give the dressing one last good shake and pour over the salad. Give the salad a good toss and serve immediately.