Combine dry ingredients: In a mixing bowl, stir together the all-purpose flour, whole wheat flour, baking soda, and salt. Set aside.
Combine wet ingredients: Add sourdough discard, milk and water into a separate mixing bowl and stir together. Stir in brown sugar and maple syrup.
Mix: Add the dry mixture into the wet mixture. Stir together. Add the raisins, seeds, nuts, and rosemary. Stir batter until all the ingredients are well combined.
Prepare loaf pan: Cut a square piece of parchment, at least 12-by-12-inch. Turn a loaf pan over, center the parchment over the bottom of the loaf pan. Use scissors to make four cuts from the edge of the paper to each corner. Turn loaf pan over. Use cut slits to fold edges in to fit parchment snugly into loaf pan.
Fill loaf pan: Use a spatula or spoon to add the batter to the loaf pan.
Ferment: Cover filled loaf pan with plastic wrap and leave at room temperature overnight or 6 to 10 hours to ferment and rise.
Preheat: After the fermentation time, preheat oven to 400º F.
Bake: Place pan into oven and bake 40 to 42 minutes, until a toothpick or knife inserted into the center comes out clean.
Cool: Allow the loaf to cool completely for 3 to 5 hours. Do not rush this step. It is ideal if bread is completely cooled and set, once cooled place in the fridge to make the loaf firmer for slicing.
Second preheat: Preheat oven to 325º F.
Prepare baking sheets: Get out two baking sheets. Cut two large pieces of parchment. Place each piece on a baking sheet.
Slice: Use a sharp serrated bread knife to cut 1/8 to ¼ inch thick slices of bread. For a tip—use heavy weights like a bag of flour or pots as a makeshift vice to hold the loaf firmly in place when you are slicing. Lay slices onto the parchment lined baking sheets.
Second bake: Toast slices 15 minutes, then flip and toast 10 to 18 more minutes until they are golden brown.
Cool: Remove baking sheets from oven and allow the slices to cool on the baking sheet.
Serve: Serve the Twice-Baked Seeded Raisin Breads with cheese or package to gift to a friend.