1. Melt butter for 40 seconds on high.
2. Add flour and cook for 1 minute on high.
3. Stir in milk. Cook 3-4 minutes on high,until sauce boils.
4. Stir in sour cream.
5. Blend sauce with avocado until smooth.
6. Dissolve gelatine in water and cook for 20 seconds on high.
7. Fold dissolved gelatine, prawns, mustard and lemon juice into avocado mixture. Season to taste.
8. Spoon into a serving dish and chill 3-4 hours.