1 teaspoon sesame seeds, toasted
Grate the zest of 1 orange and the lime into a large bowl. Squeeze 1 tablespoon of juice from the grated orange and 2 tablespoons of juice from the grated lime into the bowl (you may need a second lime to get the 2 tablespoons of juice). Whisk in the soy sauce and the oil. Set the dressing aside. Segment the remaining 3 oranges.
Use a sharp knife to slice the rind and pith from the remaining oranges. Working over a mixing bowl, cut between the membranes to free the orange segments, letting them drop into the bowl. Pour any juice into a cup and reserve it for another use.
Add the mache, frisée, and cucumbers, if using, to the orange segments.
Using a large sharp knife, cut the salmon into 1/2-inch-thick slices. Add the salmon to the bowl. Drizzle with the dressing, and toss gently to coat. Allow the flavors to meld for at least 2 minutes.
Divide the salad among 4 serving plates. Sprinkle with the toasted sesame seeds, and serve.