Measure Water (1 1/4 cup) in a measuring cup. Add to this the Active Dry Yeast (1 tablespoon) and Granulated Sugar (1 teaspoon). Use a fork to stir it around for a few seconds. Let the mixture set for 5 minutes.
Meanwhile, in a large mixing bowl, combine the All-Purpose Flour (2 cup), Whole Wheat Flour (1/2 cup), and Salt (1 1/2 teaspoon) briefly with a spoon or fork.
Pour water and yeast mixture into your stand mixer with a dough hook attachment. Set on low speed, gradually add the flour mixture until emptied. Mix the dough for 10 minutes in total, to allow for it to be kneaded by the dough hook. Scrape the sides of the bowl with a spatula occasionally if the flour isn't incorporating into the dough.
When done mixing, lightly flour your dough on the top and all sides (using a spatula to flip dough) to prevent excessive sticking. Cover the bowl with a towel and let rise for 1 hour.
While dough is resting, prepare the banneton by generously flouring the inside of the basket. Make sure to get it into the creases and use your fingers to press it onto the sides. Be patient and gentle so the flour stays on the sides. Sprinkle the Oatmeal (1 1/2 teaspoon) into the basket.
When the dough has risen, liberally flour your work surface and gently transfer your dough to the surface with a spatula. It is a moist dough, so you'll want to flour your hands too. Fold the dough in half like a taco. Then fold in half the opposite sides. Fold in half 2 more times.
Place the loaf seam-side down into your banneton. Cover with a towel. Let rise for 30 minutes.
While the loaf is rising, place an empty dutch oven with its lid on into a 460 degrees F (240 degrees C) oven for at least 30 minutes.
After the loaf has risen, take the dutch oven out of your oven with mitts and remove the lid. Tilt the bread dough basket gently and quickly to transfer the dough into the dutch oven. Be careful! The dutch oven and lid is extremely hot!
Put the lid on the dutch oven and place it back into the oven to bake for 30 minutes. After 30 minutes, remove the lid and bake for an additional 10 minutes.
Take out the dutch oven and remove the bread with a spatula or similar tool, placing on a wire rack to cool. Slice and serve.