In a bowl, combine flour and yeast. Add water and whisk well to avoid lumping. Cover tightly and leave at room temperature overnight.
Set oven to 350°F and proof box to 105°F with 80% humidity. In a heavy bottom pan, heat olive oil and caramelize onion to a deep golden brown. Cool and set aside.
In an upright mixer fitted with a dough hook combine flour, yeast, sugar, shortening, and salt. Add starter and water and mix/knead on low for 10 minutes. Ensure that dough is forming a ball and isn't leaving any unmixed ingredients in the bowl. If needed, scrape down bowl and increase speed for the remaining time.
Stop mixer and let dough rest for 5 minutes. Knead on low for an additional 5 minutes. Remove from mixer and divide into workable sections. Cover each with a clean, warm, damp cloth. Allow dough to double in size at room temperature, about 40 to 60 minutes.
In a bowl, combine turkey skin, onion powder, salt, garlic powder, poultry seasoning, and pepper. Mix well to combine. Set aside until needed.
Punch down dough and divide sections into 250 gram portions. Roll each portion into a ball and place on a lightly floured parchment-lined sheet tray. Allow to rest and increase in size at room temperature for about 30 to 40 minutes.
Flatten each dough ball and roll out to a roughly 12" by 8" rectangle. If dough is tight and elastic it may need to rest once in the rolling process.
Evenly spread each piece of rolled out dough with 19.2 grams (2 tablespoons) hydrated shallots. Flip dough over and roll out back to the 12" by 8" rectangle.
Spread this side with 14 grams (1 tablespoon) caramelized onion and sprinkle with 15.5 grams (2 tablespoons) seasoned turkey skin. Roll up from one of the long edges into a tight roll and place seam side down into a 12" hoagie form.
With a sharp knife, score rolls at a 45° angle and spray lightly with milk. Proof until doubled in size, about 1 hour. Move rolls from proof box to oven.
Bake until rolls are light golden brown and feel slightly hollow when tapped, about 18 minutes. Remove from oven and allow to cool. Serve as desired.
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