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1
In a small nonreactive saucepan, combine the onion, balsamic and sherry vinegars, sugar and 1/4 teaspoon of cracked black pepper and bring to a boil over high heat.
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2
Reduce the heat to moderately low and cook, stirring occasionally, until the liquid is reduced to a thick syrup and the onions are soft, about 30 minutes.
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3
Serve the marmalade chilled or at room temperature.
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4
Meanwhile, preheat the oven to 425.
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5
In a medium saucepan, cook the broth over high heat until reduced to 1 cup, 10 to 15 minutes.
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6
In a large ovenproof skillet, melt 1 tablespoon of the butter in the olive oil over moderately high heat.
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7
When the foam subsides, season the pork lightly with pepper and cook in the skillet until browned on 3 sides, about 2 minutes per side.
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8
Transfer the pan to the oven and roast the meat for 8 to 9 minutes, or until an instant-read thermometer inserted in the thickest part reads 135.
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9
Transfer the pork to a cutting board; let rest while you finish the sauce.
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10
Meanwhile, discard the fat in the skillet and return the pan to high heat.
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11
Add the reduced broth and boil over high heat, scraping up any brown bits from the bottom of the pan, until reduced to 1/4 cup, about 6 minutes.
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12
Off the heat, whisk in the remaining 3 tablespoons of butter.
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13
Season with salt.
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14
Slice the tenderloin across the grain and arrange on 4 plates.
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15
Spoon some of the onion marmalade next to the pork, drizzle with the pan sauce and serve.
