In a large bowl, toss 450g shelled, deveined medium prawns, 2 tbsp olive oil, 1 tbsp fresh lemon juice, 3 crushed garlic cloves, 2 tsp Dijon mustard and 1 tsp hot paprika. Refrigerate for 10 minutes. Meanwhile, thinly slice 1 head each cos lettuce and radicchio, and transfer to a large serving bowl. In a small bowl, whisk ¼ cup plain fat-free yoghurt, 3 tbsp each fresh lemon juice and finely grated Parmesan cheese, 1 crushed garlic clove and ¼ tsp each salt and pepper; toss with lettuce mixture in bowl. Spread prawns on a foil-lined baking sheet and roast at 200°C for 10 minutes or until pink; add to salad along with 1 cup prepared unseasoned croutons.