Preheat the oven to 180oC, gas mark 4. Mix the garlic and oil in a mixing bowl, add the croutons and toss in the oil mixture. Season and spread over a foil-lined baking tray. Roast for 15 minutes, stirring halfway.
Meanwhile, for the dressing, mix the yogurt, mayonnaise, Parmigiano Reggiano, Worcestershire sauce, lemon juice and anchovy with 2-3 tsp water until you have a smooth dressing. Season with black pepper.
3 Arrange the lettuce leaves, prawns, avocado and croutons on plates. Spoon over the dressing and scatter with extra cheese shavings.