Preheat your oven to 350° Fahrenheit and prepare your spring form pan. To do this, line the bottom with parchment paper and grease the whole thing with either cooking spray or butter.
Sprinkle the brown sugar into your pan. Then cover the entire bottom with sliced strawberries and rhubarb. Get creative and use whatever pattern you'd like!
Add all purpose flour, polenta, salt and baking powder to a medium sized bowl and whisk to combine. Then set aside.
In a medium sized bowl, whisk together granulated sugar and butter until light and smooth. Then add your eggs and lemon zest and mix again. Now add the dry mixture and mix until combined. Add the milk and mix. Finally, add the diced strawberries and rhubarb and mix once more.
Pour the batter into your prepared pan. Smooth out the top using a spatula. then place on a rimmed baking sheet and bake for 50-60 minutes or until you can stick a toothpick in it and it emerges clean.
Once it's done baking, place the pan on a cooling rack and let cool for at least 20 minutes. Slide a knife around the sides of the cake to loosen it. Then release the springform pan, and place a platter or cake stand face down atop the cake. Carefully flip the cake (use a towel or oven mitts if it still feels hot). Peel off the parchment paper, and enjoy!
Notes
Cake can be stored in an airtight container at room temperature or in the fridge for a few days.