*Note: Medium oatmeal may be used instead of oat flakes if preferred. Mix the oats, flours, cheese and a pinch each of salt, paprika and cayenne. Cut then rub in the fat, bind with water and leave out to rest. Peel, quarter and slice the onions wafer thin. Turn them in melted butter, cover and leave to sweat over very gentle heat for 20 minutes or so until meltingly tender, just stirring occasionally. This pastry is rather difficult to handle so it is best to divide it in half and roll out in two batches. Keep the other half covered meanwhile. Use the pastry to line little mincepie tins (or individual 4-inch fluted flan tins if you prefer). Don't worry too much if the pastry tears; patch it together by pressing it with your fingertips. Blind bake on a pre-heated baking sheet at 400 F (200 C) gas mark 6, allowing 12 minutes with greaseproof paper and beans, then 12 minutes without the lining. When the onions are beautifully soft, remove the lid and cook over increased heat for several minutes until they are a rich golden brown. Season generously with salt and black pepper, add a dash of cayenne and paprika and mix well. Then tip the contents of the pan into a sieve placed over a bowl to drain off the buttery juices and to cool the onions slightly. When the pastry cases are cooked, beat the eggs with the soured cream, add onions and check seasoning. Spoon the mixture into the tarts, sprinkle the grated cheese on top and bake at 400 F (200 C) gas mark 6 for 20-25 minutes until the filling is deliciously puffed up and golden.