For the compote, combine rhubarb, raspberries, sugar, lemon zest and juice in a heavy bottom saucepan over medium heat. Bring to a hard boil, stirring, and let simmer for 10 minutes. Add cornstarch or tapioca starch, and stir to dissolve. Let cook for 3 minutes longer, until mixture has thickened slightly. Stir in vanilla. Remove from heat and let cool for 15 minutes.
Meanwhile preheat oven to 350 degrees F. In a large mixing bowl, whisk together oat flour, oats, brown sugar, tapioca starch, baking powder, sea salt and cinnamon. Stir in butter and mix well so that no white streaks remain.
Using the back of a spatula or spoon, firmly press a heaping cup of oat mixture into a parchment lined 8x8 baking pan. I find a spoon works really well to press it together and get it smooth.
Prick crust with fork a few times. Bake for 12 minutes, until crust is fragrant and lightly browned. Remove from oven and let cool for 10 minutes.
Top with 1 1/4 cups of rhubarb compote, then sprinkle the top with the rest of the oat mixture. Bake for additional 25-30 minutes, or until topping is browned. Set aside to cool, then refrigerate. Then cut. They taste best this way! And it gives them the best texture, firm and sturdy.