Toast the quinoa in a medium saucepan over medium-high heat, stirring frequently, until the grains smell nutty and begin to pop, about 5 minutes.

Stir in the water and a pinch of salt. Bring to a boil, then reduce the heat to low, cover, and simmer until the liquid is absorbed and the quinoa is nearly tender, 14 to 18 minutes.

Spread the quinoa onto a rimmed baking sheet and let it cool completely — this allows the residual heat to finish cooking the grains gently so they stay fluffy.

Whisk the lime juice, jalapeño, cumin, and a half-teaspoon of salt together in a large bowl. Slowly whisk in the olive oil until the dressing is emulsified, then stir in the cilantro.

Add the cooled quinoa, bell pepper, mango, black beans, and scallions to the bowl with the dressing. Toss gently to combine and season with salt and pepper to taste.

Arrange the avocado slices over the top and serve at room temperature.