Preheat oven to 350° F/175 C. Grease a 9” x 5”/23cm x 13cm loaf pan and line with parchment with over hang in at least one direction for easy removal.
In a large bowl, combine flour, baking powder, baking soda, sugar, and salt. Stir to evenly distribute.
Make a well in the center and add oat milk, oil, and vanilla. Stir until mostly incorporated.
Pour in the lemon juice and zest and stir until completely smooth. You have to work fast from this point because the acid will start reacting with the baking soda immediately.
Scrape into prepared pan and level out.
Bake for 45-55 minutes, until golden brown on the edges, set in the center and skewer inserted comes out clean.
Allow to cool in the pan for at least 20 minutes before transferring to a cooling rack.
When the loaf is mostly cool, make the glaze.
Optional: for a richer lemon flavor, brush the loaf with 2 tablespoons of warm lemon syrup made from equal parts lemon juice and sugar (heated until dissolved). Leave it to soak before adding the additional sugar glaze/frosting,
In a measuring cup or small mixing bowl with a spout, combine the powdered sugar with just enough lemon juice to make a thick but pourable frosting,
Drizzle over the loaf and allow to set.
Best the same day or can be stored in an airtight container in the fridge for up to 3 days (but the glaze will get sticky). Bring to room temperature before serving.