Preheat the oven to 175 degrees Celcius.
Line or grease a spring form.
Add all the ingredients for the vanilla custard to a small pot (start by only adding 1 tablespoon of corn starch and add 1/2 tablespoon at a time more if needed). Let it shimmer on low heat, stirring regularly to avoid any lumps in the custard.
Taste to check if more vanilla or sweetener is needed.
When the sauce has thickened turn off the heat and let it cool a little while you prepare the cake batter.
Add stevia granular sugar, coconut oil, apple sauce to a high speed blender and mix until a little fluffy and smooth.
Add eggs, yogurt and stevia vanilla drops and mix again, just until combined.
In another bowl combine oat flour, rice flour, almonds flour, baking soda and salt and stir until combined.
Add the dry ingredients to the wet and run in the blender on low speed, just until combined.
Add the vanilla custard and stir using a spatula until incorporated in the batter.
Add the diced rhubarb and strawberries and fold in using a spatula.
Add the batter to the spring form. Top with rhubarb stems and slivered almonds.
Bake in the middle of the oven for 25-35 minutes, until an inserted toothpick comes out clean.
Let the cake cook for at least 30 minutes allowing it to set.
Top with stevia icing sugar.