DO NOT combine the ingredients for the main dough, instead just add them to the 2 pre-doughs (optionally add cooked onion now).
70 grams water, 9 grams salt, 175 grams dark rye flour, 3 grams active dry yeast
Knead by hand or using the dough hook of a mixer until you have a homogenous, barely sticky dough and all ingredients are worked in (about 5 minutes).
Cover airtight and let rest at room temperature for 1 hour. The dough should have increased by about half of its size.
Shape the dough into either a round or a long shape, depending on the shape of your basket (I personally prefer the longer shape usually because I like smaller, more even slices). You can watch my video to help you understand the shaping process. Generally speaking, you will fold the top of your dough over to the bottom, then press down. Repeat from all sides and work all 'flaps' into the middle.
Prep a proofing basket (Brotform) with white flour (I don't use the liner), then add the shaped loaf seam side down. Cover with a linen towel or the liner of the basket, and let proof (final rise period) at room temperature for 45 minutes.
Preheat your oven to 250° Celsius (480° Fahrenheit) and add a casserole dish filled with hot water to the bottom of your oven.
Prep a baking sheet with parchment paper, then flip your dough onto it. Check to make sure there is still plenty of water in your casserole dish. Transfer quickly to the hot, steamy oven and bake for a total of 60 minutes (reduce the temperature to 220° C/ 430° F 10 minutes into baking).