Preheat oven to 375 degrees F.
Into a glass measuring cup, add unsweetened almond milk and lemon juice. Set aside. The milk will thicken and curdle slightly as it stands; this is normal.
Into a medium-size mixing bowl, add whole wheat flour, rolled oats, dark brown sugar, baking soda, baking powder, and sea salt. Stir to combine the dry ingredients.
Into a large mixing bowl, add almond milk/lemon juice mixture, melted butter, vanilla bean paste, and Irish stout beer. Stir gently to combine.
In the large mixing bowl, stir the dry ingredients into the wet ingredients until fully incorporated. The batter will be thin.
Into a prepared 9" x 5" loaf pan, pour all the batter.
Bake, uncovered, for 35 minutes.
Then, without removing the bread from the oven, reduce the temperature to 325 degrees F.
Continue baking for an additional 35 minutes.
Leaving the bread in the oven, turn off the heat completely and open the oven door. Allow the bread to cool in the oven for 30 minutes.
Remove bread from the oven then turn out onto a wire rack.
Serve warm or room temperature.