Preheat the oven to 425 degrees F. Line a muffin pan with muffin cups or grease the muffin pan with butter and set aside.
In a large bowl, stir together the spelt flour, oats, cinnamon, baking soda, baking powder and salt. In a medium bowl or a large glass measuring cup, whisk together the applesauce, eggs, oil, honey, milk, and vanilla extract.
Add the wet mixture to the dry mixture and stir to combine. Add the chopped rhubarb and fold until completely incorporated and no dry patches of flour remain.
Fill each muffin cup to the top and sprinkle additional oats on top of each (optional). Bake at 425 degrees F for 5 minutes, then lower the temperature to 350 degrees F and bake for an additional 15 minutes or until a toothpick poked in the centre of a muffin comes out clean. Broil for 2 - 3 minutes to brown and crisp the muffins tops (optional).
Remove from the oven and let the muffins cool for at least 10 minutes in the muffin pan before removing to finish cooling on a wire rack.