Preheat oven to 425° F / 220° C. Line a 12-cup muffin tin with silicone or paper liners.

In a large mixing bowl, whisk together rolled oats, all purpose flour, cornmeal, light brown sugar, baking powder, ground cinnamon, kosher salt, and ground nutmeg.

In a separate bowl, whisk together melted butter, almond milk, egg, lemon zest, lemon juice, and vanilla extract.

Make a well in the dry ingredients. Add wet ingredients and use a wooden spoon or rubber spatula to stir until just combined. Do not overmix. (Batter will be lumpy.)

In a small bowl, toss berries in 1-2 tablespoons of flour. With a rubber spatula, fold berries into batter.

Spoon batter into prepared muffin tin, dividing it equally between the 12 cups.

Sprinkle with oats and Demerara sugar.

Bake in preheated oven for 23-25 minutes or until muffins are golden brown and nicely risen, and spring back lightly when touched.

Transfer to a wire rack to cool. Serve warm or at room temperature.

Store leftover muffins in an airtight container at room temperature for up to three days. Mixed berry muffins can also be frozen.