Preheat the oven to 400° F (200° C) and line 12 muffin cups with paper liners.
Bring the lentils and oats to a simmer in a medium saucepan with 1 cup of water.
Whisk together the flour, sugar, baking powder, cinnamon, and salt in a large bowl. Once the oatmeal and lentils have cooled to a lukewarm temperature, stir in the milk, egg, and oil. Add this mixture to the dry ingredients along with the blueberries (and any other fruits or nuts you like) and stir just until combined; do not worry about getting all the lumps out of the batter.
Fill the paper-lined muffin cups and bake for 20 minutes, until pale golden and springy to touch.