Put the rhubarb and pear into a 1.2 litre (2 pint) ovenproof pie dish with half the sugar.
Put the remaining sugar in a food processor, add the flour and butter and process until the mixture resembles fine breadcrumbs. Alternatively, put the ingredients into a bowl, add the butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Stir in the marzipan.
Spoon over the fruit and sprinkle with a few flaked almonds. Cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 35–40 minutes until golden brown, checking after 15–20 minutes and covering with foil to prevent over-browning if necessary. Serve hot with custard.