In a large bowl, pour ½ cup hot water over the rolled oats. Let them sit for about 10 minutes to soften. This prevents the oats from pulling moisture from the dough later. Do not drain it off.
Add the butter, honey, salt, active sourdough starter, and remaining water to the bowl of soaked oats. Stir until combined. Gradually mix in the flour until a shaggy dough forms. The dough should feel soft but not overly sticky.
Knead the dough until smooth and elastic. You can knead by hand or use a stand mixer with a dough hook. Mix on low (speed 1–2) until combined — about 2–3 minutes. Then, increase slightly (speed 2–3) and knead for 8–12 minutes until the dough comes together.
Place the dough into a greased bowl, cover with a tea towel or plastic wrap, and let it bulk ferment at room temperature for 4 to 6 hours, depending on your kitchen. The dough should rise noticeably and feel airy. You are looking for a 40–60% rise, not a full double.
Turn the dough out onto a lightly floured surface. Gently shape into a loaf and place into a greased 9×5 inch loaf pan.
Cover and let it rise again for 2 to 4 hours, or until the dough has risen just above the rim of the pan. Once risen and if using, brush the top of the loaf with egg wash and sprinkle with oats. If you prefer to slow ferment the loaf overnight, you can place the shaped dough directly into the refrigerator after putting it in the loaf pan. Cover it tightly and refrigerate for 8 to 12 hours. The next day, remove the pan from the refrigerator and allow the dough to sit at room temperature until it rises just above the rim of the pan. Depending on how much it rose overnight and the temperature of your kitchen, this may take 30 minutes to 2 hours. Bake once it looks pillowy and slightly domed over the top.
Preheat your oven to 375°F. Bake for 40 to 55 minutes, or until the loaf reaches an internal temperature of 195°F. Let cool completely before slicing.