1. Toast quinoa in a large saucepan over medium-high heat, stirring often, until quinoa is very fragrant and makes continuous popping sounds, 5 to 7 minutes. Stir in water and 1/2 teaspoon salt and bring to simmer. Cover, reduce heat to low, and continue to simmer until quiona has absorbed most of the water and is nearly tender, about 15 minutes. Spread quinoa in a rimmed baking sheet and set aside until quinoa is tender, all water has been absorbed, and quinoa has cooled, about 20 minutes.

2. Meanwhile, process 1/4 cup lime juice, jalapeño, cumin, and 1 teaspoon salt in blender until jalapeño is finely chopped, about 15 seconds. Add oil and cilantro and continue to process until smooth and emulsified, about 20 seconds. Transfer 1/4 cup vinaigrette to small bowl and stir in remaining 1 tablespoon lime juice; cover.

3. When quinoa is cool, transfer to a large bowl. Stir in bell pepper, mango, black beans, and remaining vinaigrette; cover.

To Make Ahead

Refrigerate quinoa mixture and vinaigrette separately for up to 2 days. To finish and serve: Remove quinoa mixture and vinaigrette from refrigerator and let sit at room temperature for 15 minutes. Whisk vinaigrette to recombine, then drizzle over salad. Stir in scallions and season with salt and pepper to taste. Arrange sliced avocado over top and serve.