In a large mixing bowl, add the raw and mashed potatoes. Whisk until combined. Add the egg, egg white, both cheeses, half and half cream, and oil, and mix until completely incorporated.
In a separate large bowl, combine the flour, baking powder, garlic powder, and salt, stir to combine. If you’re adding cayenne pepper (for a kick), add it now.
Stir the dry ingredients into the wet ingredients until a soft dough is formed. Remove the dough onto a lightly floured flat space (cutting board or counter top) and kneed 6 -7 times.
Gather the dough up and form a ball, placing it in an oiled 8 inch cast iron pan.
Using a sharp knife, cut an x on the top of the dough. Brush with butter, and sprinkle with fresh chopped rosemary. Bake for 45-55 minutes, until golden brown on top.
Remove from oven and cool for 10 minutes. Run a butter knife around the edges of the pan, and remove the loaf from the pan.
Continue to cool completely before slicing.
Notes
Optional to use both raw and mashed potatoes, or you can use 1 ½ cups of leftover mashed potatoes.