1. Preheat oven to 180 C. Melt 30g of butter in a pan over medium heat. Add onion and gently cook till soft and translucent (about 10 minutes). Transfer to a small oven proof dish.
2. Cut the fish into 4 portions and lay on top of onion mixture, spooning a little of it over the fish. Lightly season the fish and pour over the cream. Cover with a lid or parchment/tinfoil. Place in oven for 15-20 minutes until fish is cooked.
3. As fish is cooking melt the remaining butter over low-med heat. Add breadcrumbs and a little seasoning. Gently toast the crumbs until slightly golden and crunchy. Transfer to a plate to cool before mixing in cheese.
4. Remove fish from oven 5 minutes before it is done. Top with the defrosted peas and crumb mixture. Return to oven for 5 minutes.
5. Serve with any of the following: your favourite green vegetables, new baby potatoes, creamy mash, wholemeal pasta, fluffy quinoa, leafy or vegetable salad or crusty sourdough bread.