The topping is a streusel. Early recipes on both sides of the Atlantic used only flour, sugar and butter. Oats, nuts and coconut came into the recipes after the Second World War. Apple and rhubarb are the standard fillings, and it is served with custard, cream or ice cream.
The dish is young. It does not appear in the first Fannie Farmer cookbook of 1896. The earliest known apple crisp reference is in Good Housekeeping in September 1915, and the earliest known apple crumble recipe is in the Canadian Farmer’s Magazine of February 1917. Alan Davidson, in The Oxford Companion to Food, found no early recipes and took crumble for a wartime invention.
In wartime Britain, when butter and fats were rationed, the Ministry of Food circulated crumble recipes made with margarine. After the war the crumble became what Hugh Fearnley-Whittingstall has called a “national institution.”