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Heat a medium size pot over high heat. When hot, add in the butter and the oil. Swirl around to coat bottom of pan.
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Once the butter is melted, sprinkle in the onion. Once it starts to sizzle, turn heat down to medium low. Stir periodically for 20 minutes.
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Sprinkle in the brown sugar, Worcestershire sauce and salt. Stir again periodically for 10 minutes.
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Let cool before use, then store in a covered dish in the refrigerator.