1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with non-stick spray.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, brown sugar, baking soda, cinnamon, nutmeg, ginger, and salt until well combined. Make sure there are no lumps of brown sugar.

3. In a separate medium bowl, whisk together the sourdough discard, buttermilk, vegetable oil, egg, and vanilla extract until smooth. The mixture should be uniformly blended with no streaks of egg yolk.

4. Pour the wet ingredient mixture into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and will result in a more tender muffin. Overmixing can lead to tough muffins.

5. Gently fold in the grated carrots, and if using, the chopped walnuts or pecans and raisins. Distribute them evenly throughout the batter.

6. Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. You can use an ice cream scoop for easy and consistent portioning.

7. Bake for 20-25 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back lightly when gently pressed.

8. Allow the muffins to cool in the muffin tin for 5 minutes before transferring them to a wire rack to cool completely. This helps prevent them from getting soggy on the bottom.