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Preheat oven to 450 degrees fahrenheit
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Season pork loin with rosemary, thyme, garlic powder, salt and pepper
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Heat cast iron skillet to Medium-High heat
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Add 2 tbsp butter and enough balsamic to coat the pan (2-3 tbsp)
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Cook pork loin over Medium-High heat for 3 minutes on each of the larger-surface area sides, and 2 minutes on each of the smaller sides (10 minutes total)
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Add red onions to the skillet. Transfer cast iron skillet to oven (carefully!) to bake for 15 minutes
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After 15 minutes, take out skillet and place back on stovetop. Use tongs to move the pork loin to a plate or surface and cover in foil
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While pork is resting, heat skillet up to Medium and add 2 tbsp butter. Gradually stir in 1/3 cup balsamic vinegar, scraping any bits and spices from the bottom and stirring for about 5 minutes
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Finish pork by adding back to the pan and coating with the balsamic sauce