1. Place the onion quarters curved side downwards on a heat-proof serving plate and arrange in a pattern radiating outwards, like petals of a flower. (If the onions are very large, cut them into 8 segments.)
2. Mix the mustard seeds together with the softened butter and spread this mixture over the onions.
3. Cover with cling film and microwave on HIGH for 6 minutes.
4. Garnish with parsley and serve immediately.