Cook the barley according to the package directions. Spread on a plate and refrigerate until cool.
Meanwhile, heat 1 teaspoon of the oil in a large cast-iron or other nonstick skillet over medium heat.
Season the salmon with ¼ teaspoon salt and ⅛ teaspoon pepper and cook until opaque throughout, 3 to 5 minutes per side. Transfer to a plate and refrigerate until cool.
Using a fork, flake the salmon into bite-size pieces.
Cut away the peel and pith of the oranges: working over a small bowl, cut along both sides of each orange segment, releasing the segments into the bowl.
Whisk together the vinegar, the remaining 2 tablespoons of oil, 2 tablespoons of the accumulated orange juice, ¼ teaspoon salt, and 1/8 teaspoon pepper in a large bowl.
Add the spinach, cilantro, avocado, orange segments, scallions, and cooked barley and toss gently to combine. Serve topped with the salmon.