Preheat your oven to 325°F (170°C).
Spray an 8½×4½-inch loaf pan with cooking spray.
In a large bowl, whisk together the stone-ground whole wheat flour, all-purpose flour, kosher salt, and baking soda until well combined.
Make a well in the center of the flour mixture.
In a medium bowl, whisk together the buttermilk, molasses, and egg.
Add the buttermilk mixture to the well in the flour mixture. Using one hand like a claw, gently mix the wet ingredients into the dry ingredients.
Spoon the dough into the prepared pan, and spread it until even.
Sprinkle the top with steel-cut oats.
Bake until deep golden brown and an instant-read thermometer inserted in the center registers 200°F (93°C).
Let cool in the pan for 15 minutes, then remove from the pan; wrap in a clean tea towel to cool on a wire rack.
