Heat the oven to 180C/160 C Fan
If your traybake tin is non-stick you do not have to grease and line it. If you have a traditional pan then grease it well, and line the base with parchment paper. I like to line the tin anyway because it makes it easier to remove the cake from the tin.
Wash and finely chop the rhubarb, set aside.
Add all the ingredients except the rhubarb, demerara sugar and flaked almonds, to a stand mixer bowl, or a large baking bowl.
Mix on low speed until the mixture comes together, then mix on medium speed for 2 minutes. Scrape down the sides occasionally to ensure all the ingredients mix well. If mixing by hand, beat with a wooden spoon until the mixture is a smooth batter.
Stir in the chopped rhubarb and spoon the mixture into the traybake tin (pan).
Scatter the demerara sugar and flaked almonds evenly over the cake mixture.
Bake for 45 - 50 minutes until a skewer inserted into the centre of the cake comes out clean.
Leave in the tin to cool completely (about an hour) , then cut into 16 portions.