Preheat your oven to 325°F (163°C). Grease and flour a 12-cup bundt pan.

In a stand mixer or large mixing bowl, cream together unsalted butter and sugar until light and fluffy, about 3-5 minutes. Add large eggs and vanilla extract, beating until smooth and creamy.

In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the butter mixture, alternating with buttermilk, starting and ending with dry ingredients. Mix on low speed until just combined, fold in lemon zest and half of the fresh blueberries.

Pour the batter into the prepared bundt pan, scatter remaining blueberries over the top. Bake for 60-80 minutes, or until a toothpick comes out clean.

Once baked, let the cake cool on a wire rack for at least 20 minutes.

For the crumble topping, combine oats, chopped almonds, and sugar in a bowl. Toast in the oven for 5-10 minutes at 325°F.

To create the glaze, whisk together buttermilk and powdered sugar until smooth. Drizzle over the cooled cake and sprinkle with the toasted crumble topping.