In a large mixing bowl, combine the active sourdough starter and warm water. Stir until the starter is mostly dissolved.

Add the bread flour, salt, and diastatic malt powder (if using) to the bowl. Mix with a wooden spoon or your hands until a shaggy, cohesive dough forms and no dry flour remains. Do not overmix.

Cover the bowl with a clean kitchen towel or plastic wrap and let the dough rest for 30 minutes (autolyse).

After the autolyse, lightly oil your hands. Perform 3-4 sets of 'stretch and folds' over the next 2-3 hours, with 30-45 minute rest periods between each set. To do a stretch and fold, grab a section of the dough, stretch it upwards, and fold it over itself. Rotate the bowl and repeat 4-6 times until all sides have been stretched and folded. The dough will become smoother and more elastic with each set.

After the stretch and folds, cover the bowl again and let the dough bulk ferment at room temperature (around 70-75°F / 21-24°C) for a total of 4-6 hours. The exact time will depend on your starter's activity and room temperature. The dough should become noticeably puffy, increase in volume by about 30-50%, and have a few small bubbles on the surface.

While the dough is fermenting, prepare your raspberry swirl. If your jam is very thin, you can gently heat it in a small saucepan over low heat, stirring in 1 tsp of cornstarch until thickened. Let it cool completely before using. The jam should be thick enough to spread without being too runny.

Lightly flour your work surface. Gently turn the dough out onto the surface. Divide the dough into 8 equal pieces, approximately 120-130g each, using a dough scraper.

Take one piece of dough and gently flatten it into a rectangle, about 6x4 inches. Spread about 1 tablespoon of the cooled raspberry jam over two-thirds of the dough rectangle, leaving one long edge clear.

Starting from the raspberry-covered long edge, tightly roll the dough towards the clear edge. Pinch the seam firmly to seal. Gently roll the log back and forth to create an even cylinder about 8-9 inches long.

To form the bagel shape, overlap the ends of the dough log by about 1 inch. Moisten one end slightly with a drop of water and firmly pinch the ends together to seal, forming a ring. Alternatively, poke a hole in the center of the log and gently stretch the dough to create a ring. Ensure the seal is strong to prevent it from opening during boiling and baking.

Place the shaped bagels on a baking sheet lined with parchment paper, ensuring they don't touch each other.

Cover the baking sheet loosely with plastic wrap to prevent drying. Refrigerate for 12-24 hours. This cold proof develops flavor and makes the bagels easier to handle.

Preheat your oven to 425°F (220°C). If you have a baking stone or an inverted baking sheet, place it in the oven while it preheats for a crispier bottom crust.

Bring a large pot (8-quart or larger) of water to a rolling boil. Add the baking soda and barley malt syrup (or honey) to the boiling water.

Gently remove the cold bagels from the refrigerator. Using a slotted spoon or spider, carefully drop 2-3 bagels into the boiling water at a time, ensuring not to overcrowd the pot. Boil for 60 seconds per side (total 2 minutes per bagel).

Remove the boiled bagels with the slotted spoon, allowing excess water to drain. Place them back on the parchment-lined baking sheet (you may need a fresh sheet if the previous one got too wet).

Immediately transfer the baking sheet(s) to the preheated oven. Bake for 18-22 minutes, or until the bagels are golden brown and cooked through. If using a baking stone, carefully transfer the bagels directly onto the stone for baking.

Once baked, transfer the bagels to a wire rack to cool completely before slicing and serving. This allows the internal crumb to set.