In a 12-inch nonstick skillet, heat oil over medium-high heat until very hot. Season salmon generously with salt and pepper; add to skillet top-side down and cook, undisturbed, until deep golden, about 4 minutes. Turn and sear on opposite side until just barely cooked through, 1 1/2 to 3 minutes longer, depending on thickness.
While salmon is cooking, stir orange zest, juice and red onion into lentils. Divide salad among 4 plates, top with salmon and serve.