Mix the flour, sugar, salt and yeast in a bowl. Mix the egg, milk and butter, add to the flour mixture and knead for approx. 10 mins. to form a soft, smooth dough. Cover and leave to rise at room temperature for approx. 2 hrs. until doubled in size. Divide the dough into 8 portions, shape into balls, place on a sheet of baking paper, cover and leave to rise at room temperature for approx. 30 mins.
Mix the rhubarb, raspberries and sugar in a wide pan, bring to the boil, then boil for approx. 4 mins. while stirring. Allow the rhubarb mixture to cool slightly, puree and leave to cool.
Fill a pot to ⅓ with oil and heat it to approx. 160°C. Flatten the dough balls by hand before deep frying. Using a slotted spoon, slide them into the pan in batches (bottom side up), deep-fry for approx. 4 mins. on each side. Remove and drain on paper towels.
Insert a straw two-thirds of the way into the doughnuts, work the straw backwards and forwards a few times until the hole is approx. 5 mm wide. Transfer the jam to a piping bag with a smooth nozzle (approx. 5 mm in diameter) and pipe into the doughnuts. Dust the doughnuts with icing sugar.