Preheat your oven to 375 F/ 190 C. Line a 12-cup muffin pan with paper liners.

Sift the flour, baking soda and salt together in a small bowl; set aside.

In a stand mixer fitted with the paddle attachment (or by hand with a wooden spoon): cream the vegan butter, sugar, vanilla extract and zest together. Incorporate the flax.

Add sourdough starter and water, stir to incorporate, let sit for at least 10 minutes.

Add dry ingredients and chopped walnuts, stir until combine.

Fold the blueberries into the bowl with a rubber spatula. Make sure to evenly distribute the berries throughout the batter (check the bottom of the bowl!).

Fill the muffin pan, about ¾ of the way full. Top with a few berries if you like, making sure to tuck them in so they don’t pop out during baking.

At this point, you can bake right away or chill overnight, for 12+ hours.

To bake now: place the tin on the center rack and bake for 25-28 minutes.

To bake later: remove the tin from the refrigerator; discard the foil. Bake straight from cold for 25-28 minutes.

Cool the muffins in the pan for 15 minutes. Transfer to a wire rack to finish cooling so the bottoms don’t get soggy.

Serve warm or at room temperature with powdered sugar.