The day before, heat the olive oil in a non-stick frying pan, then cook the salmon, skinned side down, for 12-15 minutes, turning half way through. It should be cooked all the way through and opaque in the centre.
Meanwhile, make the dressing. Put olive oil, mustard, honey, mayonnaise and vinegar with some salt and pepper into a blender and whizz until well combined, then set aside.
Peel and segment the oranges and remove any pith (work over a plate so you can catch all the juices). Cover the oranges and keep in a cool place. Stir the orange juice into the dressing and store it in a screw top jar. Slice the onions, put into a plastic bag and store in the fridge.
Flake the salmon into large pieces using two forks to tease the flesh apart, then spoon over half the dressing. Cover and chill.
When ready to serve, shake the dressing, put the sliced onion, prawns and orange segments in a large bowl and drizzle over the remaining dressing. Add the salad leaves and toss together lightly. Using your hands, gently lift the mixture on to a large platter and scatter over the salmon flakes. Season with some coarsely ground black pepper and serve immediately.