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

Combine water, sugar, and yeast in a small bowl; let sit until yeast is creamy, about 5 minutes. Stir yeast mixture into flour mixture until well mixed and caraway seeds are evenly distributed. Cover the bowl with plastic wrap and let sit at room temperature for 18 hours.

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

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.

Carefully place dough in the preheated 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 an oven rack and bake for 5 minutes.