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Melt butter in a large pot over medium-high heat.
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Once melted, add onions and cook onions until they become translucent, about 10 minutes.
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Sprinkle onions with sugar; reduce heat to medium.
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Cook, stirring constantly, until onions are soft and browned.
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Stir sherry into onion mixture and scrape bottom of pot to dissolve brown bits from the pot.
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Transfer onion mixture into a crock pot and add in beef broth.
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Season to taste with seasoning salt; stir in thyme.
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Cover crock pot and cook on High for 4 to 6 hours or Low for 8 to 10 hours.