For the caramelized onions: In a medium pot, add the butter and set over medium-high heat. Add the onions and sprinkle with salt, stirring once to coat, and reduce the heat to medium-low. Allow to slowly caramelize, until dark golden brown and the edges are crisped, 12 to 15 minutes, stirring only a few times during the process. Season and remove from the heat. Set aside.
For the dressing: Mix the yogurt with the cilantro, lime juice, shallots and agave nectar. Season with salt and pepper. Refrigerate until ready for use.
For the salad: Combine the edamame, snow peas and tomatoes in a large mixing bowl. Add the dressing and the caramelized onions and toss thoroughly coat. Season with salt and pepper.
To serve, spoon the salad into a serving dish.