Lightly season the tuna with salt and pepper and coat with sesame seeds on all sides. Shake lightly to remove the excess. In a frying pan, heat 30ml of oil and sear the tuna, 3 minutes per side. Remove from heat and cut into 6mm slices.
To make the vinaigrette, in a small bowl, mix the remaining olive oil, along with the balsamic vinegar and lime juice. Season with salt and pepper and whisk vigorously. Set aside.
To make the salad, in a medium bowl, toss the avocado, watercress, red onion, and sweet pepper.
Place the salad on serving plates, arrange the tuna, and drizzle with the vinaigrette.