A naturally leavened loaf of toast that makes a great base for sandwiches. Describing the steps I'm assuming some experience with sourdough baking and won't go into detail about stretch and fold and steam baking etc. Happy to answer questions in the comments.
I've done this in a variety of loaf pans, the exact size doesn't matter.
The rough timeline is mixing/kneading, three hour proofing with stretch and fold, shaping, another three hours of undisturbed proofing, baking. —Vera Kern
Combine all the ingredients but a small portion of the water into the bowl of your stand mixer, fitted with the dough hook. Mix on low, scraping down the sides if needed and add more of the water if needed. Knead for about 5 minutes, then increase speed to medium and knead for another 5 minutes.
You can do this by hand as well. Just mix all the ingredients in a big bowl with a wooden spoon until they've come together and then knead by hand.
The dough should show some gluten development and come away from the sides of the bowl but doesn't need to pass the window pane test.
I like to then take the dough out of the bowl and knead it for a few minutes by hand to make sure it's smooth.
Place dough in a bowl, which can optionally be slightly oiled or buttered. Cover and let proof for three hours. During this time do a set of four stretch and folds after one and after two hours.
After the dough has proofed for three hours, prepare your loaf tin by greasing it with butter. I've used a 8.5x4.5 in and a 9x5 in pan and both worked well.
Put the dough on the counter and shape however you like so it fits into your pan. You can do this as one big piece or divide the dough, shape smaller loafs and sit them in the pan together. You can also do a braid. Anything goes as long as it fits into the pan.
Losely cover the pan and let the loaf proof for another three hours.
Half an hour before the three hours are up, start preheating your oven to 425 F/220C with a baking stone/steel if you have one.
For the first part of the bake, we'll need some steam so prepare for that as well. (Baking tray in the bottom of the oven, spray bottle with water, dutch oven large enough to hold your loaf pan)
Brush the top of the loaf with egg wash and sprinkle with poppy or sesame seeds.
When ready to bake, decrease the temperature to 400F/205C and bake the loaf in the pan with steam for 10 minutes. Then bake for 15 minutes without steam. After that you can either continue baking it in the pan for another 30 min or take the loaf out of the pan and continue baking it on the baking stone/steel or a baking tray.
Let loaf cool on a wire rack. Wait at least an hour before you cut into it.