Preheat your oven to 350°F (175°C). Grease a 9x5-inch loaf pan.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, salt, ground cinnamon, and nutmeg until well combined.
In another bowl, mix the melted butter, milk, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix; a few lumps are okay.
In a small bowl, combine the brown sugar, ground cinnamon, and nuts (if using) for the swirl.
Pour half of the batter into the prepared loaf pan. Sprinkle half of the cinnamon swirl mixture over the batter.
Pour the remaining batter on top and finish with the rest of the swirl mixture. Use a knife to gently swirl the mixture into the batter.
Bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.