1. Heat a large heavy-bottomed skillet over medium-high heat. Dry the pork medallions well and season with salt and pepper. Add the oil to the skillet and heat until shimmering. Lay the pork in the pan and cook until browned on the first side, about 4 minutes. Turn and cook until the second side browns, about 3 minutes more. Transfer the pork to a plate, and cover loosely with foil.
2. Add the onions to the skillet, stir and season with salt and pepper. Cover, and reduce heat to medium and cook until soft, about 8 minutes. Add the cider vinegar and stir, scraping up the browned bits that cling to the skillet. Add the onions to the pork.
3. Add the 1 tablespoon butter to skillet, raise heat to medium-high. Add the apples and sage; cook, stirring occasionally, until golden, about 3 minutes. Stir in the broth and mustard, and simmer until the apples soften, about 2 minutes. Return the onions and any accumulated juices from the pork to the skillet. Simmer to thicken slightly, about 2 minutes. (If desired swirl in up to 2 tablespoons of butter to thicken the sauce). Nestle the pork in the onions and heat until just warmed through. Divide the pork among 4 plates and top with the apples and onions. Serve.
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