Make vinaigrette

In a bowl, whisk together lime juice and zests and add oil in a stream, whisking until emulsified.

Peel mango and cut into julienne strips.

Pat salmon dry and cut crosswise into 1/3-inch thick slices. Season salmon with salt and pepper. In a 12-inch non-stick skillet, heat oil over moderately high heat until hot but not smoking and sear salmon in batches, 15 seconds on each side, or until just cooked through. With a metal spatula (I use a wide-bladed putty knife - it's thinner than a spatula and works great with fish), transfer salmon as it's cooked, to a plate. Add vinegar to skillet and simmer over moderate heat until syrupy, about 3 minutes. Return salmon to skillet, turning to coat with balsamic reduction.

In a large bowl, toss spinach with vinaigrette and salt and pepper to taste. Mound spinach on 4 plates and top with salmon and mango.