Prep pan and oven. Position a rack in the center of your oven and preheat the oven to 350°F. Lightly grease a 9 inch by 5 inch loaf pan. I like to line mine with parchment paper as well and grease lightly again.
Combine Dry Ingredients. In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, pumpkin pie spice, cinnamon, and nutmeg.
Combine wet ingredients. In a second mixing bowl, whisk together the oil, sugars, eggs, vanilla, pumpkin, milk and sourdough discard.
Combine wet and dry ingredients. Make a well in the center of your dry ingredients and pour in the liquid ingredients. With a spatula, stir to combine until the last of the flour has been mixed in. Stop mixing at this point to avoid overmixing.
Bake the bread. Spoon the batter into the pan. Bake the bread for 60-70 minutes, or until a cake tester or toothpick inserted in the center of the loaf comes out clean. You should see moist crumbs, not raw batter. Mine was perfect at 60 minutes, so begin checking your bread at 60 minutes, but every oven is different so you may need additional time.
Cool the bread. Remove the bread from the oven, and cool it on a rack in the pan for 10 minutes. Remove from the pan and continue to cool. When it's completely cool, wrap it well in plastic wrap, and store it overnight before serving for best texture.