Tip the flour, yeast, spices, chilli, coriander and a large pinch of salt into a bowl and stir together. Whisk in the lager until you have a smooth batter. Cover and set aside while you cook the chips.
Heat oven to 180C/160C fan/gas 4. Drizzle half the oil in a roasting tin and put in the oven to heat up. Tip the chips into a bowl and toss in the remaining oil, curry powder and a pinch of salt. Tip into the roasting tin and roast for 30 mins, then toss and carry on roasting until the chips are cooked through and crisp, around 15 mins more. Turn the oven down to 110C/90C fan/gas ¼ to keep them warm.
Stir the onion into the batter, making sure all the slices are separated. Heat 10cm sunflower oil in a deep-fat fryer or pan of oil to 180C. Scatter some flour on a plate, dip the fish in the flour, pat off any excess, then dip into the batter, coating the fish with a generous amount of batter and onion. Carefully lower into the oil and sizzle for 5 mins until golden brown. Once cooked, drain off any excess oil, then carefully place on a tray lined with kitchen towel. Serve with the chips, seasoned with curry salt and lime mayo on the side for dipping.