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1
In a large saucepan, combine 5 cups (1.2L) stock with reserved shrimp shells and mushroom trimmings. Bring to a gentle simmer and cook for 20 minutes. Strain and return stock to saucepan.
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2
Meanwhile, combine shrimp, 3/4 teaspoon (3g) salt, baking soda, and cornstarch in a medium bowl and toss to coat. Refrigerate for at least 30 minutes and up to 1 hour.
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3
Whisk grits into stock, set over medium-high heat, and bring to a simmer, whisking frequently. Lower heat to a bare simmer and cook, stirring and scraping bottom frequently with a wooden spoon, until grits are fully softened and cooked and have thickened into a spoonable porridge, about 1 hour. Stir in Gruyere cheese until fully melted. Season with salt and pepper and keep grits warm. (A piece of parchment pressed against the surface will help prevent a skin from forming.)
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4
In a large skillet, heat bacon over high heat until sizzling. Lower heat to medium and cook, stirring, until bacon has rendered its fat and become crisp, about 5 minutes. Using a slotted spoon, transfer bacon to paper towels to drain. You should have 1/4 cup rendered bacon fat in the skillet. Remove all but 1 tablespoon fat and reserve.
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5
Return skillet to high heat and heat until very lightly smoking. Add shrimp and cook, stirring frequently, until lightly browned outside with only a faint trace of translucence remaining in the center of each shrimp. Transfer shrimp to a plate.
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6
Add reserved 3 tablespoons bacon fat to skillet, return to medium-high heat, and heat until shimmering. Add mushrooms and cook, stirring, until mushrooms release their liquid, about 3 minutes; scrape any browned bits from bottom of skillet. Continue cooking mushrooms, stirring frequently, until lightly browned, about 6 minutes; if pan becomes too dry, add vegetable oil, 1 tablespoon at a time, to keep it lubricated. Stir in shallot, garlic, and cayenne and cook until shallot and garlic are softened, about 2 minutes.
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7
Add remaining 3/4 cup (180ml) stock and scrape up any browned bits on bottom of pan. Stir in shrimp. Lower heat to medium-low and whisk in butter until fully melted and emulsified with the sauce. Remove from heat and whisk in lemon juice. Stir in herbs and season with salt and pepper.
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8
Spoon grits into bowls and top with shrimp, mushrooms, and their gravy. Top with reserved crispy bacon and additional herbs for garnish, then serve right away.
