Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch cast iron skillet.
Pour hot water over raisins and currants in a bowl and let soak. Whisk flour, sugar, salt, and baking soda in a separate bowl until well combined; whisk in buttermilk.
Drain raisins and currants and pat dry with paper towels; fold into dough. Turn dough out onto a floured work surface and knead gently until it holds its shape; if dough is too sticky, knead in more flour. Form into a round and place into prepared cast iron skillet. Use a sharp knife to cut a cross into the top of the loaf.
Bake in the preheated oven until golden brown and a slender knife inserted into the loaf comes out clean, about 1 hour.
