The night before you make your pancakes, mix the nondairy milk with the vinegar and set aside for a couple of minutes to curdle.
In a large bowl, mix the nondairy milk with the sourdough starter, flour, and maple syrup or sugar. Cover with plastic wrap and keep it in a warm place (like an unheated oven with the light turned on) overnight.
By morning, your pancake batter should be all puffy and bubbly and it should have risen. Mix in the salt, baking soda, flaxmeal, blueberries, and lemon juice, if using.
Heat a griddle and coat with cooking spray. Pour the batter on the griddle using a 1/4 cup measure for each pancake.
Cook until the sides are dry and bubbles appear in the center, then flip and continue to cook on the other side until set.
Serve hot with maple syrup and fresh blueberries, if desired.