Fish and chips with tartare sauce

Name: Fish and chips with tartare sauce
Ingredients: 2 egg yolks; 1 tbsp Dijon mustard; 3 tbsp white wine vinegar; 200ml/7fl oz vegetable oil, plus extra for deep-frying; squeeze lemon juice; ½ red onion, chopped; 1 tbsp gherkins, roughly chopped; 1 tbsp capers, roughly chopped; small handful parsley, finely chopped; 4–5 green olives, chopped; 12 tbsp plain flour; 6 tbsp cornflour; 200ml/7fl oz ale; pinch salt; 8 large Maris Piper potatoes, peeled and cut into chunky chips; 4 x 150g/5½oz cod (or haddock) fillets