Step 1

Mix rye flour, bread flour, buttermilk, caraway seeds, vital wheat gluten, and kosher salt together in a very large bowl.

Step 2

Combine water, sugar, and yeast in a bowl until yeast softens and begins to form a creamy foam, about 5 minutes. Stir yeast mixture into flour mixture until well mixed and caraway seeds are evenly distributed. Cover bowl with plastic wrap and let sit for 18 hours.

Step 3

Transfer dough to a heavily floured work surface. It will be somewhat elongated, so take the right and left sides and fold them into the middle. Turn dough over and gently use a spatula to tuck corners under; cover with plastic wrap for 15 minutes. Remove plastic wrap, dust dough with flour, and make shallow cuts in the top for full bloom.

Step 4

Place a Dutch oven in the oven and preheat oven to 500 degrees F (260 degrees C).

Step 5

Carefully place dough in the Dutch oven, cover immediately, reduce oven temperature to 460 degrees F (238 degrees C), and bake until bread is cooked through, 30 to 35 minutes. Carefully transfer bread from the Dutch oven to the oven rack and bake for 5 minutes more.