Preheat your oven to 375°F (190°C). Prepare a muffin tin by greasing it well or lining it with paper liners.

In a large bowl, whisk together all-purpose flour, sugar, baking powder, salt, cinnamon, and nutmeg until well combined.

In a separate bowl, whisk together milk, oil, egg, and vanilla extract until smooth and creamy.

Gently fold the wet mixture into the dry ingredients using a spatula. Add in the diced peaches and fold them in just until evenly distributed.

In a small bowl, mix cold butter, brown sugar, and flour together using a fork until the mixture resembles coarse crumbs.

Spoon the muffin batter into the prepared muffin cups, filling each about two-thirds full. Sprinkle the streusel topping over the batter.

Place the muffin tin in the preheated oven and bake for 20-25 minutes until golden brown.

Allow the muffins to cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely.