Griddled asparagus with prawns & rouille

INGREDIENTS

• 350g asparagus , or 40 spears
• 4 tbsp olive oil , plus extra for brushing
• 800g large raw prawns , thawed if frozen
• zest and juice 1 lemon
• small handful basil leaves , torn
• 3 handfuls young salad leaves , to serve
• 1 egg
• 2 tsp white wine vinegar
• ½ tsp Dijon mustard
• 100ml each olive and sunflower oil
• 1 garlic clove
• ½ tsp cayenne pepper
• pinch saffron