Preheat the oven to 200C/400F/Gas 6.
To make the white sauce, melt the butter in a frying pan. Add the garlic and onion and fry for 2-3 minutes, until soft.
Add the peas and cook for five minutes.
Add the double cream and simmer for 3-4 minutes, until thickened, then season with salt and freshly ground black pepper.
To make the haddock, rub the haddock with the oil and season well on both sides with salt and freshly ground black pepper.
Heat an ovenproof frying pan and when very hot, add the haddock, skin-side down, to fry for one minute, until golden. Turn the haddock and transfer to the oven to roast for 6-8 minutes, or until completely cooked through.
To make the onion rings, heat the oil in a deep saucepan until a breadcrumb sizzles and browns when dropped in it. (CAUTION: hot oil can be dangerous. Do not leave unattended!)
Place the flour in a large bowl, add the sparkling water and whisk vigorously to form a batter.
Dip the onion rings in the batter then carefully place into the oil to deep-fry until crisp and golden. Carefully remove with a slotted spoon and drain on kitchen towels.
To serve, spoon the sauce into the middle of a warm plate. Place the haddock onto the sauce and top with the onion rings.