Preheat the oven to 375℉. Grease and line a 9 inch springform pan with parchment paper. Set aside.
In a medium mixing bowl, sift together the spelt flour, almond flour, baking powder, baking soda, and salt. Whisk to combine.
Add the softened butter to the bowl of an electric mixer. Using the paddle attachment, cream the butter until smooth. This will take about one minute. Add the coconut sugar and beat until combined. Add in the eggs and beat one final time until smooth.
Use a baking spatula to fold the vanilla extract, almond extract, Greek yogurt, and lemon zest into the wet mixture.
Pour the dry ingredients into the wet and fold in using a baking spatula. Mix until no dry bits of flour remain.
Transfer the batter to the prepared springform pan. I like to use a large cookie scoop to do this, but this is not necessary. Smooth out the batter so it is evenly spread across the pan.
Slice the rhubarb stalks into pieces that are approximately 2 inches long and place on top of the cake batter. I did a bit of a chevron pattern, but you can do any placement you like. Gently press the rhubarb into the batter.
Mix the melted butter and maple syrup in a small bowl. Brush the top of the cake generously with the butter mixture. Transfer the cake to the preheated oven and bake for 45 minutes or until a toothpick inserted in the centre comes out clean.
Allow the cake to cool for 20 minutes in its pan. Remove the cake from the pan and allow to cool for a further 10 minutes.
Serve the cake warm with a cup of coffee (or tea!). The cake is best enjoyed the day it is baked, but will keep stored at room temperature in an airtight container for up to four days.