Preheat the oven to 200°C.
Melt butter in a pan and fry the garlic and onion for 3 minutes until soft.
Add the peas and cook for 5 minutes.
Stir in double cream, simmer until thickened, and season.
Rub the fish with oil and season both sides.
Fry fish skin-side down in a hot ovenproof pan for 1 minute, then roast in the oven for 8 minutes.
Heat vegetable oil in a deep saucepan.
Whisk flour and sparkling water together to create a batter.
Dip onion rings in batter and deep-fry until golden, then drain on towels.
Serve the fish on the sauce topped with onion rings.