Combine yeast and warm water in a small bowl and let sit for 10 minutes until foamy. Melt butter in a small saucepan over low heat, set aside to cool.
In the bowl of a stand mixer fitted with a dough hook*, combine flour, raisins, walnuts, brown sugar, cinnamon, and salt. Pour the water and yeast mixture and the melted butter into the bowl and stir until just combined. The dough will be sticky and wet; you do not need to knead it. Cover with plastic wrap and let sit at room temperature for 8-24 hours. The dough will rise significantly and look puffy when it's ready to bake.
To bake, place a Dutch oven with lid in the oven and preheat to 450°F. This allows your Dutch oven to preheat as well.
While the oven heats, transfer the dough to a lightly floured surface. Using a bench scraper and your hands, gently shape until a ball is formed, dusting with more flour if necessary. The dough will still be very soft. Transfer the dough to a piece of parchment paper and let sit until the oven is heated.
Carefully remove the Dutch oven from the oven and transfer the dough (still on the parchment paper) into the Dutch oven (remember, this will be hot!). Cover with the lid and bake for 35 minutes. Remove the lid and bake an additional 10-15 minutes. Lightly tent with aluminum foil if you feel your loaf is browning too quickly. When finished, the bread should be browned, crusty, and sound hollow when tapped. A thermometer inserted in the center of the loaf will read 190°F.
Immediately remove the bread from the Dutch oven and the parchment paper to cool completely on a cooling rack. Allow the loaf to cool fully before slicing.
Notes
*You do not need a stand mixer to make this recipe. You can also make the dough in a large mixing bowl and mix with a wooden spoon.