Preheat your oven to 400°F (200°C). Line a standard 12-cup muffin tin with paper liners, or lightly grease each cup. If using, combine 1 tablespoon granulated sugar and 0.25 teaspoon ground cinnamon in a small bowl for the topping and set aside.

In a large mixing bowl, whisk together the flour, 0.5 cup granulated sugar, baking powder, 0.5 teaspoon salt, and 0.5 teaspoon ground cinnamon until well combined.

In a separate medium bowl, whisk together the milk, vegetable oil, large egg, and vanilla extract until smooth.

Pour the wet ingredients into the dry ingredients. Stir with a spatula or wooden spoon just until the flour mixture is moistened. The batter will be lumpy, and that's perfectly fine – avoid overmixing!

Gently fold in the diced nectarines. Stir just enough to distribute them evenly throughout the batter.

Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. If using the cinnamon-sugar topping, sprinkle a small amount over the top of each muffin.

Bake for 18-22 minutes, or until the muffins are golden brown and a wooden skewer inserted into the center of a muffin comes out clean. Baking at a slightly higher temperature initially helps create that lovely domed top.

Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy warm or at room temperature!