Heat the oil in a large skillet set over medium-high heat. Season the prawns with salt and pepper and cook 1 to 2 minutes per side, until just cooked through. Spoon the prawns and cooking oil into a wide, shallow bowl and cool to room temperature.
Place the lentils in a fine sieve. Rinse with cold water, and then allow the lentils to drain several minutes. Place the lentils in the bowl with the prawns and mix in the pesto, lemon juice, orange juice, cumin, cucumber and red pepper or red onion. Cover and refrigerate mixture until ready to serve. (Can be made an hour before needed.)
When ready to serve, divide and mound the salad greens on 4 plates. Top with the prawn/lentil mixture. Divide and top salad with the feta cheese and enjoy.
Note: To devein a prawn, after peeling it use a small paring knife to make a lengthwise slit along its back. Pull out, or rinse out with cold water, the dark vein. Pat the prawn dry.