Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with parchment liners or lightly grease with olive oil.
In a small bowl, combine 2 tablespoons of ground flaxseed with 5 tablespoons of water. Stir well and set aside for 5 minutes to thicken.
In a large bowl, whisk together flour, baking powder, baking soda, salt, chopped rosemary, and black pepper.
In a separate medium bowl, whisk together olive oil, plant milk, maple syrup, apple cider vinegar, and the prepared flax eggs until smooth.
Make a well in the center of your dry ingredients and pour in the wet mixture. Fold gently with a spatula just until combined – about 10-12 strokes. Don't overmix.
Fold in the diced apples, walnuts (if using), and nutritional yeast with just 3-4 more gentle strokes. The mixture should look lumpy but have no visible dry flour pockets.
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full. For a professional touch, sprinkle a few extra apple pieces and a tiny sprig of rosemary on top of each muffin.
Bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean with just a few moist crumbs.
Allow the muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.