Preheat your oven to 400 degrees Fahrenheit and place a rack in the middle position. Line a 12-cup muffin tin with paper liners or lightly spray each cup with cooking spray to prevent sticking.

In a large mixing bowl, whisk together the all bran cereal, whole wheat flour, sugar, baking powder, baking soda, salt, cinnamon, and nutmeg to evenly distribute the leavening agents.

In a medium bowl, whisk the egg, milk, vegetable oil, and vanilla extract until combined and smooth, taking care not to overmix.

Pour the wet ingredients into the dry ingredients and gently stir with a wooden spoon or spatula until just barely combined. The batter should remain lumpy and thick to prevent dense muffins.

If using, fold in the raisins or dried cranberries with a couple of gentle strokes to evenly distribute them.

Divide the batter evenly among the 12 muffin cups, filling each about two-thirds full to allow for domed tops without overflow.

Bake the muffins in the preheated oven for 20 to 24 minutes, until the tops are light golden brown. Watch carefully to avoid overbaking.

Test for doneness by inserting a toothpick into the center of a muffin; it should come out clean or with a few moist crumbs. If wet batter remains, bake for an additional 1 to 2 minutes and test again.

Allow the muffins to cool in the tin for 5 minutes to prevent them from sticking or collapsing, then transfer to a wire rack to cool completely before serving.