Using the bowl of a countertop mixer, add 10-grain cereal and 1 cup of water; stir until well combined. Let mixture sit for 30 minutes.

Using a countertop mixer fitted with a dough hook, add whole wheat flour, all-purpose flour, butter, honey, yeast, salt, eggs and remaining 1 cup of warm water. (Note: The egg yolk and oats will be used later.)

Mix on low speed for 1-2 minutes, or until mixture comes together. Increase speed to medium and mix for 5 more minutes. (Note: Dough will be a little sticky at this point.)

Transfer dough into a large greased bowl. Cover bowl with plastic wrap and place in a warm location until approximately doubled in size (~75 minutes).

Turn dough out onto a lightly-floured cutting board. Gently press air out of the dough and fold dough several times.

Divide dough into 2 equal portions. Working with one piece of dough at a time, flatten dough into 8” square. Fold dough into thirds (similar to folding a letter) and place into 9”x4” baking pan. Repeat process with remaining piece of dough.

Spray tops of dough with nonstick baking spray and cover lightly with plastic wrap; let dough rise until approximately doubled in size (~45-60 minutes).

Preheat oven to 375°F.

Using a small bowl, whisk the egg yolk with 1 Tbsp of water. Gently brush this mixture on top of the loaves. Sprinkle oats evenly on top of the 2 loaves.

Bake for 30-35 minutes, or until bread is fully baked and tops of loaves are golden brown. (Note: If you have an instant-read thermometer, the bread will be done once the internal temperature reaches 200°F.)

Let loaves cool in pan for 10 minutes before turning them out onto a cooling rack until completely cool.