Combine boiling water, cereal mix, and quinoa; let sit for 10 minutes.
In a large bowl, combine the warm water, active sourdough start, honey, wheat flour, and 2 cups of the bread flour. Stir until smooth.
Cover and let sit for 30 minutes at room temperature. Stir in the salt and pepitas, then add 1 1/2- 2 cups more bread flour, enough to make a sticky dough.
Cover and let the dough rest for 30 minutes, then use wet hands to fold one side of the dough over to the other side. Continue stretching and folding each of the other three sides across the dough. REPEAT THIS PROCESS TWO - THREE MORE TIMES, until the dough is very stretchy.
Cover the bowl and place the dough in the fridge overnight, or for 12-15 hours.
Let the dough sit at room temperature for about an hour, then use wet hands to form it into a ball. Place it on a piece of parchment paper.
Let the loaf sit at room temperature for 2-4 hours, or until it has doubled. (This will take longer if your kitchen is cool.)
Preheat your oven and a 4 quart dutch oven to 450°. Cut slits in the top of the dough with a bread lame or sharp knife. Spritz with water and sprinkle the oats and sesame seeds over the top.
Carefully lift the parchment and dough and place in the hot Dutch oven. Place 3-4 ice cubes between the parchment and the pan.
Add the lid. Bake for 45 minutes at 450 degrees with the lid on. Remove the lid and bake for another 10-15 minutes, or until the crust is golden brown and the internal temperature is around 195°.
Carefully use the parchment to remove the bread from the pan and place it on a wire rack. Let cool for at least 20 minutes - if you can wait that long! It does slice best after it has cooled completely.