Sprinkle top of the cake with flaked almonds. Stir together the 1/2 cup white sugar and 1/2 tsp cinnamon for the topping, then sprinkle liberally over the top of the cake.
Bake in preheated oven until golden and a tester inserted in the centre comes out clean, 60-70 minutes, checking at about 50 minutes and if cake is browned enough, lay a sheet of aluminum foil loosely overtop to cook it longer without over-browning. *If starting with frozen rhubarb, the cooking time will be the closer to 70 minutes.
Allow to cool in the pan for 5 minutes, then run a knife around the outside and carefully remove the outer ring. Transfer to a cooling rack (with the bottom part of the pan still on) and allow to cool until warm. Serve immediately or allow to cool completely, then remove bottom part of pan.
Be sure to read the "Cook's Notes" in the original post, where I share some valuable tips, options, substitutions and variations for this recipe!