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Preheat the oven to 450°F. Rinse and pat dry the pork, then season all over with 1 teaspoon salt and a few grindings of pepper.
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In a large ovenproof skillet over medium-high heat, warm the olive oil. Add the pork and sear on all sides until browned, 5 to 7 minutes total. Transfer to a plate, let cool slightly, then rub all over with the mustard. Set aside.
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Melt the butter in the skillet over medium heat. Add the leeks, pears, and rosemary and sauté until golden, about 5 minutes. Distribute the mixture evenly in the skillet, season with salt and pepper, top with the pork, and roast in the oven until a meat thermometer inserted into the thickest part registers 140°F to 145°F, 16 to 20 minutes (for the best texture, don’t overcook). Transfer the pork to a plate; tent loosely with foil.
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Using a pot holder, return the skillet of pears and leeks to the stovetop. Add the broth and chard and cook over high heat until the greens are wilted, 1 to 2 minutes. Season to taste with salt and pepper.
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Slice the pork diagonally 1/2 inch thick. Spoon the pears, leeks, and greens onto 4 plates, top with the pork slices, sprinkle with salt, and serve.