In a large bowl, whisk or beat together the melted ½ cup unsalted butter and 1 cup granulated sugar. Slowly mix in the 2 large eggs, then stir in the ½ cup whole milk. Fold in the dry ingredients, mixing with a wooden spoon until just combined.
In a medium bowl, whisk together the 1 ½ cups oat flour, ½ cup almond flour , 2 teaspoons baking powder, and ¼ teaspoon fine sea salt. Reserve 2 tablespoons of this mixture, then stir the remaining dry ingredient mixture into the butter mixture.
Toss the 2 cups fresh or frozen blueberries and 2 tablespoons of flour mix in a medium bowl, then gently fold the blueberries into the batter.
Divide the batter evenly between the muffin cups and let the batter rest. Preheat the oven to 375℉.
Prepare the topping: In a small bowl, whisk together the ½ cup oat flour, ½ teaspoon ground cinnamon, and ⅓ cup brown sugar, then stir in the 3 tablespoons unsalted butter with a fork until crumbly. Sprinkle the topping evenly over the muffins.
Place in the oven and bake for 25 minutes or until a tester inserted in the middle of a muffins comes out clean. Remove from the oven and cool the muffins for 5 minutes in the pan before removing to a rack to cool completely.
Notes
Grease the muffin tin well. These are soft, did I mention that?! So make sure to really grease the muffin tin with oil or non-stick spray so that the muffins release easily. Or, use muffin liners.
Toss berries in flour. Make sure to follow the recipe instructions to toss the blueberries in a few teaspoons of the dry mixture. This will help prevent them from sinking to the bottom.
Don't substitute for a mix. I know it seems easier, but you'll truly love the texture that comes from the oat and almond flours. If you must, though, give my homemade gluten free flour blend a try.
Cool completely. As noted, these muffins are very soft straight out of the oven. Let them cool fully so that the crumb sets.
Make ahead. For a firmer texture, bake the muffins the night before you plan to serve them and store them in an airtight container on the counter.