Make the brown bread: Preheat the oven to 350º and grease a 9-by-5-inch loaf pan with butter. In a medium bowl, whisk together the cornmeal, rye and whole wheat flours, baking soda, salt and cocoa powder. In a large bowl, whisk together the buttermilk and molasses. Stir the dry ingredients into the wet ingredients until a smooth batter forms. Fold in the raisins and transfer to the prepared pan.
Place the loaf pan into a roasting pan and pour the boiling water into the roasting pan to come halfway up the sides of the loaf pan. Bake in the oven until golden brown and the bread reaches an internal temperature of 190º, 1 hour. Remove from the water bath and let cool completely before slicing.
Make the toast: In a small bowl, mix together the yogurt with the horseradish to incorporate. Spread each piece of toast with 2 tablespoons of the horseradish cream and top with 1 ounce of smoked bluefish slices. Garnish with sliced radishes, dill fronds and flaky salt, then serve.