Preheat the oven to 350F. Lightly grease a 9x9 or 8x8- inch baking dish with olive oil.
In a large bowl, whisk together the Greek yogurt, olive oil, maple syrup, vanilla, and eggs until smooth. It will be lumpy at first!
Add the oats, buckwheat flour, hemp seeds, cinnamon, baking soda, and salt. Use a rubber spatula to mix until thoroughly combined. Gently fold in the raspberries and rhubarb until evenly distributed. Transfer to the baking dish and spread out into an even layer.
Bake for 40-45 minutes, or until a toothpick, cake tester or knife in the center comes out clean. Let cool for 10 minutes before slicing.
Serve warm topped with yogurt and walnuts (if desired). For a sweeter breakfast treat, drizzle maple syrup over the yogurt. I also highly recommend eating it warm with butter.