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Heat a heavy skillet over medium heat. Add the steel cut oats; cook and stir until lightly toasted, about 5 minutes. Remove from heat. Combine bread flour, whole wheat flour, toasted steel cut oats, dry milk, sugar, gluten, salt, flax seeds, and yeast in the bowl of a stand mixer with a paddle attachment. Add 5 cups water, oil, and vinegar. Mix on low until all ingredients are moistened. Increase speed to medium-high; mix until gluten is developed and dough is elastic, about 5 minutes. Turn dough into a clean, oiled bowl. Cover with plastic wrap. Let rise in a warm place until doubled in volume, 1 to 2 hours. Stretch and fold dough 4 to 6 times. Place back in bowl, cover, and refrigerate 16 to 18 hours. Remove from the refrigerator 2 to 4 hours before baking. Turn out onto a well-floured surface; sprinkle top of dough with flour. Halve the dough; flatten each half. Fold in all 4 sides like an envelope; press edges together into the dough. Shape each piece into a loaf with the smooth side on top. Place in 2 loaf pans; cover with a non-terry towel. Let rise until dough just crests above the top of the pan, 45 minutes to 1 hour. Preheat oven to 500 degrees F (260 degrees C). Place a pan of water on the lower rack to produce steam for the bread. Whisk egg with 1 tablespoon water in a small bowl. Coat tops of loaves with the egg wash. Bake in the preheated oven until browned, about 15 minutes. Remove the pan of water carefully; cover loaves with aluminum foil tents. Continue baking until an instant-read thermometer inserted into a loaf reads 200 to 205 degrees F (93 to 96 degrees C), 45 to 60 minutes more. Remove loaves from pans and cool on a wire rack.