In the bowl of stand mixer with the dough hook attachment, combine the boiling water, steel-cut oats, salt, butter, and brown sugar and let stand 25 minutes
In a separate bowl, dissolve granulated sugar and yeast in the 1/2 cup of warm water; let stand 5 minutes or until foamy.
Add the foamy yeast to the oats. Add 2 3/4 cups all-purpose flour and 3 cups whole wheat flour to oat mixture. With dough hook attachment, beat at medium speed until well blended. Turn dough out onto a floured surface. Knead until smooth and elastic; add enough of the remaining all-purpose flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place, 1 hour or until doubled in size.
Gently press two fingers into dough. If indentation remains, dough has risen enough. Punch dough down; cover and let rest 5 minutes.
Divide in half. Working with one portion at a time (cover remaining dough to prevent drying), roll each portion into a 14 x 8-inch rectangle on a floured surface. Roll up each rectangle tightly, starting with a short edge, pressing firmly to eliminate air pockets; pinch seam and ends to seal. Place each loaf, seam sides down, in a 9-inch loaf pan coated with cooking spray. Cover and let rise 30 minutes or until doubled in size.
Preheat oven to 350ºF. Uncover dough, and brush egg evenly over loaves. Bake at 350º for 45 minutes or until loaves are browned on bottom and sound hollow when tapped. Remove from pan, and cool on wire racks.