Combine the flour and cornflour in a large bowl, season, then put 3 tbsp onto a plate and it set aside.
Gradually whisk the beer into the bowl of flours until you have a smooth, lump-free batter. Leave to rest for 30 mins while you prepare the onions and peas.
Cook the chopped onion in 1 tbsp olive oil for 5 minutes until softened. Add the peas and cook until heated through, using a fork to crush them a little. Stir in the lemon zest and mint and put to one side keeping warm.
Heat the oil in a deep saucepan until a drop of batter sizzles and crisps up straight away. Dip 16 good-sized onion rings into the batter. Carefully lower into the hot oil and fry for 2-3 minutes until golden and crisp. Lift out and drain on kitchen paper.
Pat the fish dry with kitchen paper, then lightly coat in the reserved flour mix. Shake off any excess, then dip into the batter. Carefully lower each fillet into the hot oil and fry two at a time for 5-8 minutes until golden and crisp. Carefully lift and drain on kitchen paper, seasoning with a good pinch of salt.
Serve the fish with the onion rings, peas and with lemon wedges on the side.