In a large mixing bowl, combine the all-purpose flour, instant yeast, and salt. Stir until evenly mixed.
Add the warm water to the dry ingredients and stir with a wooden spoon or spatula until a shaggy dough forms. The dough will be sticky and rough.
Fold in the shredded sharp cheddar cheese and shredded mozzarella cheese until evenly distributed throughout the dough.
Cover the bowl tightly with plastic wrap or a clean kitchen towel and let the dough rest at room temperature for 5 to 6 hours, or until it has doubled in size and bubbles appear on the surface.
After the dough has risen, preheat your oven to 450°F (230°C). Place a Dutch oven or heavy oven-safe pot with a lid inside the oven to heat for 30 minutes.
While the oven and pot heat, turn the dough out onto a well-floured surface. Using floured hands, gently shape the dough into a round loaf by folding the edges toward the center. Do not knead.
Place the dough seam-side down on a large piece of parchment paper. Cover with a kitchen towel and let rest for 30 minutes.
Carefully remove the hot Dutch oven from the oven. Lift the dough with the parchment paper and place it into the pot.
Cover with the lid and bake for 30 minutes.
Remove the lid and brush the top of the bread with melted butter mixed with garlic powder.
Return the bread to the oven uncovered and bake for an additional 15 minutes, or until the crust is golden brown and crisp.
Remove the bread from the oven and transfer it to a wire rack to cool completely before slicing.