Preheat the oven to 350℉. Grease an 8 or 9-inch cake pan and line the bottom with parchment paper.
In a large bowl, place the 1 cup granulated sugar½ cup plain full-fat yogurt, and 3 large eggs and whisk together until smooth. Whisk in the 1 ½ cups oat flour and ¼ cup almond flour, 2 teaspoons baking powder, 1 teaspoon ground cinnamon, and ¼ teaspoon fine sea salt, mixing just to combine. Add the ½ cup oil, and stir well to incorporate. Keep stirring until it forms a smooth batter. Fold in the 1 ½ cups mixed berries, then pour and scrape the batter into the prepared pan.
Rest the batter while the oven heats, then place in the oven and bake for 30-40 minutes until a toothpick inserted into the center comes out clean. Do not overbake.
Cool cake on a rack for about 20 minutes; then turn it out of the pan to cool completely, then top with a glaze if you wish.
Notes
Chop berries if using larger berries. For larger berries, such as strawberries, make sure to roughly chop them before adding the batter so you don't have huge bits of fruit in the middle of the cake.
Choose your pan. For a more decorative option, bake the batter in a bundt pan! The batter also bakes up beautifully in a round 8 or 9-inch cake pan, as shown.
Layer up. Want to make a layer cake? Double the recipe and bake it in 2 9-inch cake pans. Fill and frost the cake with homemade whipped cream or vanilla buttercream frosting.
Fresh or frozen berries? I have made this cake with fresh berries in season, or a package of mixed frozen berries. Both work great.
What type of yogurt? I prefer plain whole-milk yogurt (not Greek yogurt) in this cake. For a little extra flavor, you could use vanilla yogurt. You could also use low-fat yogurt, but I would not use non-fat because you'll lose out on the richness from the higher-fat yogurt.
Add vanilla or almond extract. For extra flavor, add 1 teaspoon of vanilla or almond extract.
Can I make it vegan? I haven't tried making this cake without eggs, but you could experiment with using flax eggs instead of eggs, and a non-dairy yogurt to make this recipe vegan.
How to serve: With your choice of a glaze, plain, sifted with powdered sugar, or a dollop of whipped cream.