Tip the rhubarb into a bowl and sprinkle over 50g of the sugar. Stir so the rhubarb is covered, then set aside for 30 mins to draw out some of the juices (macerate). Meanwhile, grease and line the base and sides of a 23cm loose-bottomed, round cake tin with baking parchment and heat oven to 180C/160C fan/gas 4.
Tip remaining sugar, the butter, orange zest and juice into a large bowl and beat with an electric whisk until well blended. Add the flour, almonds, baking powder and eggs, then beat again until smooth. Fold in the rhubarb and any juices. Spoon into the tin and level the top.
Sprinkle with the flaked almonds, then bake in the centre of the oven for 1 hr-1 hr 15 mins until risen, golden and a skewer inserted into the centre comes out clean. Cover with foil if the cake starts to brown too much during cooking. Leave in the tin for 15 mins before removing and cooling completely on a wire rack. Dust with a little icing sugar before serving.