For poolish, combine water and yeast and whisk to combine. Allow to sit a few minutes for yeast to activate. Whisk in the flour. Store the poolish in refrigerator overnight or for at least 12 hours. Note: The poolish is the starter for this recipe which helps with leavening as well as texture and flavor in the final loaves. The poolish should be used within two days of mixing because the yeast is not active enough after that.
To make sourdough bread, remove poolish from fridge and let it sit at room temperature for 30 minutes to 1 hour.
Add yeast to water and whisk well. Allow to sit for five minutes to activate yeast. Whisk in poolish and pour the wet ingredients into a stand-mixer bowl. Add all-purpose flour. Mix, for 3 minutes on speed one then increase to speed two and mix for 3 more minutes. Add salt and mix for one more minute on speed two.
If you would prefer to mix the dough by hand, follow steps one, two and three, but rather than mixing the ingredients into a mixer bowl, gradually incorporate the flour into the wet ingredients until a firm ball of dough is formed. If it is sticky, add more flour. Then, remove the dough from the bowl and place it on a lightly floured surface. Knead it by pushing your palms into the dough and then turning it one quarter. Keep kneading and doing quarter turns for about 5-10 minutes, or until the dough is supple and non-sticky.
Transfer dough to lightly oiled bowl. Cover with plastic wrap and let sit at room temperature for an hour or until doubled in size.
Separate the dough into 2 even pieces, gently round them and allow the dough balls to rest on a floured surface for 20 minutes. Cover with a damp cloth to prevent a skin from forming.
After 20 minutes, shape into batard loaf (football shape). Place on parchment lined sheet pans, keeping in mind that loaves will double in size. Spray loaves with pan spray and cover with plastic wrap. Allow to rise 2 hours or until loaves double in size.
Sprinkle every loaf with Maldon salt. Put pan on center rack and a pan of water on bottom rack. Bake bread for 8 minutes at 425 degrees. Turn oven to 325 degrees, rotate pans, and bake for 12 more minutes.
Sweat onions with oil or butter over medium heat until caramelized, about 10-15 minutes. Remove from pan and add more oil or butter.
Brown sausage in same pan until cooked thoroughly. Add caramelized onions back to pan, then drain fat. Cool completely in fridge, about 45 minutes to 1 hour.
Once onions and sausage are cooled, roll out sourdough to about a ½-inch thick, 10-inches wide, and 24-inches long.
Spread sausage and onion mixture evenly on sourdough and roll firmly, pinching the seam firmly to seal the loaf together.
Let bread proof at room temp for 30-45 minutes on a sheet tray and then place in a 400 degree oven for approximately 15-18 minutes, until golden brown and when thumped sounds a little hollow.
Let cool, then slice in 1-inch thick pieces. Brown or toast in pan or oven and top with favorite vegetables, cheese, arugula, and poached or sunny-side up egg.