Begin by preheating your oven to 350 degrees Fahrenheit. Lightly grease (with butter) AND line with parchment paper the bottoms of TWO 8-inch cake pans or THREE 6-inch cake pans.
In a small bowl, add the flaxseed meal and water together to a bowl, stirring and letting it set + thicken for 5 minutes; This makes the 'flax eggs'. In a separate bowl, add the milk + ACV together and let sit for 5 minutes until it activates and acts as 'vegan buttermilk'.
In a large bowl, whisk together the flour, oats, sugar, baking powder, baking soda, cinnamon, and nutmeg until combined.
Add in the milk-ACV mixture, melted butter, flax eggs, and mix (using a hand-mixer) until just combined. NOTE: It might not look fully combine and that’s OK-------we just need the wet ingredients to be combined together before adding them into the next step.
Add the wet ingredients in with the dry ingredients and stir until full combined. NOTE: If the batter looks extremely thick, add extra milk, 1 Tbsp at a time until it’s somewhat pourable but not runny or thin at all. Fold in blueberries.
Divide the batter evenly into each of the cake pan and bake for 20-25 minutes, or until a toothpick or butter knife comes out clean when tested in the centers.
Remove cakes from the oven and let them rest in the pan for 10 minutes or so before removing the cakes and placing them onto a cooling rack to fully cool. NOTE: Let them cool for at least 1-2 hours in a cool place.