I take a pat of butter and put it into a small Pyrex bowl and put it in the microwave for about 15 seconds. I use a pastry brush to apply a light coat of butter to each muffin well. Earth Balance or cooking oil can be used instead for a dairy free muffin. Another alternative is to use silicone muffin or cupcake forms.
At this point I set my oven to 375 degrees F. You may want to leave this step until you are ready to mix the wet with the dry ingredients.
Mix the first 5 ingredients in the list in the large bowl. A whisk makes this an easy job. It is essential that the dry ingredients are well combined.
You could combine all the wet ingredients and whisk them well but I find that it is best to whisk the egg well first then add the rest of the wet ingredients. I measure the maple syrup into a 2 cup measuring cup followed by the oat milk and then the cooking oil. As for the cooking oil I have used both canola and sunflower seed oil with equal success.
Add the fresh or frozen blueberries (and the medium chopped rhubarb if you are using it) to the dry ingredients and toss them to combine. The reasoning for doing this is to keep the berries intact. The dry ingredients coat the berries and keep their colour from bleeding through the mixture.
Create a well in the dry ingredients and add the wet. Stir just to combine leaving no dry spots. The batter may be lumpy and that is okay.
Make sure each muffin well or silicone form has some batter in it. Ideally, each should be filled 2/3 full. If you have batter left over, it won't be a lot so just top up the muffins until you have used up all the batter. An alternative is to take the Pyrex dish that had the butter in it and use it as an additional muffin form and make a baker's dozen.
In my stove I have to reverse the pan halfway through the baking process and I usually find that 19 minutes is about right. I reverse the pan after 10 minutes and bake for an additional 9 minutes.
To test doneness I insert a toothpick into a muffin at the front of the oven and if it comes out clean I insert it into one of the muffins that was in the back of the oven hoping for the same results. If I don't get them, I reverse the pan again and bake for an additional 3 minutes. See my notes on the Links, Sources & Miscellanea page for more information on my stove and for adjustments you may have to make to the temperature and baking time.