Preheat the oven to 180ºC (350ºF).
Start by browning the butter. Place the butter into a medium sized saucepan and set over a medium heat. Once melted, whisk the butter frequently to keep it moving until the milk solids turn a nutty brown colour and the butter smells fragrant. This should take around 5 minutes. Add the sugar and maple syrup to the browned butter and stir.
In a large bowl, mix together the flour, oats, bicarb and salt. Pour the butter and sugar mixture into the dry ingredients and stir to combine. The mixture will be crumbly.
Place the chopped rhubarb into a saucepan with 2 tablespoons of water. Cook over medium low heat until softened. Add the chopped strawberries and cook for another 5 minutes.
Pour half of the brown butter oat mixture into a 9x9 square brownie tin lined with greaseproof paper or a non-stick liner. Press down with your hands or the back of a spoon to compact. Pour the strawberry and rhubarb mixture over the top and spread out evenly. Finish by pouring the remaining brown butter oat mixture over the top and spread it out. Press again to compact and then place in the oven. Bake for 25 minutes until golden brown and firm to the touch.
Allow to cool in the tin. Remove from the tin and cut into 16 squares.