Grease your muffin tins. If your tin isn't non-stick then consider using paper muffin cases
Mix the ground flaxseed and oat milk in a small bowl and set aside until the flaxseed has bloomed and the liquid has thickened
Meanwhile, cut the butter into cubes and cream it together with the sugar until it has formed a smooth buttery paste with a light golden colour. This should take a few minutes in a stand mixer, or about 5 minutes by hand
Keep the sugar butter mixture in the mixing bowl and whisk in the liquid flax egg mix, before mixing in the sourdough discard and vanilla. Whisk until it has emulsified (formed a uniform liquid)
Sieve the plain flour into a mixing bowl. Using a hand whisk (or fork), whisk in the spelt flour, polenta, baking powder, bicarbonate of soda, and salt until fully incorporated
Put your mixer on to a slow speed with the wet ingredients still inside the bowl and gradually spoon in the flour mix until it has fully incorporated. If you are doing this by hand, add a larger chunk of flour at a time
Cut your rhubarb into small chunks. Get a large spoon (metal works best) and fold the rhubarb into the batter until it is evenly dispersed
Evenly spoon the muffin mixture into your muffin tin and bake in the oven for 20 to 30 minutes
Remove the muffins from the oven once they are cooked through, you can check with a small metal skewer - if any mixture sticks to the skewer when poking it into a muffin then keep it in for another 5 minutes or so. The tops of the muffins should look a lovely golden brown colour
Notes
If you don't have any rhubarb then you can easily swap it for another fruit. I really enjoy using blueberries and raspberries