Add whole wheat flour, all purpose flour, sea salt and yeast into a large bowl and mix to combine. Stir in 1½ cup water with a wooden spoon until no dry flour is visible. Add more water 1-2 tbsp at a time as needed.

Cover the bowl in plastic wrap, and leave the mixture at room temperature for 12-20 hours. The mixture should be bubbly, spongy and wet.

Heat a 5L casserole dish with lid in the oven at 400°F -450°F for 30 minutes. While you wait, dust your counter top with flour VERY generously. Scrape the spongy flour mixture onto the floured counter, dust the top of dough with some flour, and gently shape the dough into a roughly round shape. Place the dough onto a large piece of parchment paper. Once the casserole dish is heated, carefully place the dough with parchment paper inside, and cover with lid.

Bake in the oven for 30-35 minutes. Remove dish from oven and carefully lift the bread out (the dish will be very hot!). Tap bottom of the bread to make sure it sounds hollow (that means it’s cooked!). Place bread back into the casserole dish, and bake uncovered for additional 15 minutes to brown the top.

Remove bread from casserole dish, and let cool before serving.