For the dressing, in a small bowl, whisk together 45 ml lime juice, 30 ml olive oil, and the mustard. Season with salt and pepper to taste. Set aside.
In a large pan, heat the remaining olive oil over medium heat and sauté the prawns for 2 minutes. Season with the remaining lime juice and salt and pepper to taste. Set aside.
In a large bowl, carefully toss the greens, avocados, cheese, red sweet pepper, walnuts, and reserved prawns. Serve with the mustard dressing.