Adjust oven rack to the middle position and pre-heat oven to 350 degrees. Lightly coat an 8 1/2 by 4 1/2 inch loaf pan with vegetable oil spray or butter. Sprinkle 3 tablespoons of the parmesan cheese evenly over the bottom of the pan.
In a large bowl, whisk the flour, baking powder, salt, cayenne, and black pepper together. Add in 3 tablespoons of the parmesan cheese and the cheddar cheese, and mix so that there are no lumps. In a small bowl, whisk the buttermilk, oil, egg, mustard, and garlic until well incorporated. Gently fold the buttermilk mixture into the dry ingredients being careful not to over mix. (The batter will be thick).
Scrape the batter into the prepared pan, and smooth the top. Sprinkle the remaining parmesan cheese evenly over the top of the batter. Bake until golden brown, and a toothpick inserted comes out clean, about 45 to 50 minutes.
Let the bread cool in the pan for 5 minutes, then remove and cool on a cooling rack for at least 1 hour before serving. ATK recommends not cutting this bread while warm, as the texture improves as it cools.