Mix together the poolish ingredients until the mixture looks like thick pancake batter. Leave this to ferment for 2-3 hours at room temperature or in a warm place (in the oven with the light on or on the proof setting should work). Once the mixture gets bubbly, put it in the fridge immediately and leave to ferment overnight. If you are making the poolish on the same day as the bread, leave it to ferment at room temperature for a bit longer (3-4 hours), then mix the dough. (But leaving it in the fridge overnight will give the final bread a better flavor, so we would recommend that!)
The next day, take the poolish out of the fridge 1 hour before mixing the dough. Let it come to room temperature (it should get even more bubbly as it warms up).
After waiting 1 hour, mix together the sugar, yeast, and lukewarm milk and water in a mixing bowl, or the bowl of a stand mixer. Stir together and let the yeast "bloom" for 5-10 minutes: you want to see it form bubbles on the top of the liquid. If there's no change at all, your yeast may be expired and you'll want to start over with new yeast.
Add the poolish (quick tip: use a wet spatula/dough scraper/hand to handle the sticky poolish) and the rest of the dough ingredients to the bowl. Add more bread/all-purpose flour if the dough is too sticky.
Knead for 10-12 minutes until very smooth and elastic, either by hand on a clean surface or using the dough hook attachment on the stand mixer.
Cover and let the dough proof for about 2 hours, or until doubled in size and passes the poke test (the dough should NOT spring back immediately when poked, should spring back slightly but leave an indentation).
Oil a loaf pan (or line with parchment paper if you're like us and your loaf pans are the opposite of non-stick).
To shape the sandwich loaf, dump the dough out on a clean surface. Using your hands, stretch it out into a rectangle, with a width equivalent to the length of your loaf pan. The length of the dough rectangle does not matter so much.
Start rolling from one end, keeping the roll very tight.
Once you have rolled it up, pinch together any creases and roll the loaf against the countertop lightly to create surface tension and help close the crease on the bottom of the loaf.
Place the loaf in the pan and cover, letting this rise for another hour, or until doubled in size and passes the poke test again. In the meantime, preheat your oven to 350 F (175 C).
Once the dough is done rising, uncover and brush the top with a little bit of milk.
Bake the loaf for 35-40 minutes, or until the top is a light golden brown and the bread sounds hollow when you tap it.
Let cool at least 20 minutes before slicing. Enjoy!