First, gently mash the fresh blackberries and set aside. Don't over-mash them, just break them apart slightly. You can also cut the blackberries in half if you prefer larger blackberry pieces in your muffins.
Next, preheat the oven to 350F and fill a muffin tin with 8 cupcake liners.
Add the oat milk, lemon juice, lemon zest, applesauce, olive oil, and sugar to a mixing bowl. Whisk to combine.
Next, add the all-purpose flour to the mixing bowl. Add the baking powder, baking soda, and salt on top of the flour. Then, use a whisk or spatula to GENTLY mix the batter just until all streaks of flour are gone. A lumpy batter is okay at this point!
Add the mashed/chopped blackberries into the batter and gently fold in. You don't want to over-do this step or the batter will turn completely purple, so just 1-2 quick folds is perfect.
Use a cookie scoop to add the batter to the 8 muffin liners until each is about ¾ full of batter.
Then, add the muffins to the oven and bake for 20-22 minutes until cooked through and beginning to brown on top.
Remove the muffins from the oven and immediately remove from the muffin tin and place onto a wire cooling rack. Do not let the muffins cool in the tin or they risk getting soggy on the bottom.
Once the muffins have cooled, make the lemon glaze. Add the sifted powdered sugar to a bowl and pour in the lemon juice and lemon zest. Whisk together until a smooth, yet thick, glaze forms.
Pour the glaze onto the muffins and serve with fresh blackberries on top. Enjoy!