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In a pot over medium high heat add the olive oil then sauté the onions until translucent
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Add in the tomatoes, Guinness, sugar, mustard, and pepper then reduce to medium low heat
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Simmer together for 15 minutes, stirring often
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Add in the vinegar then in a food processor or blend smooth the mixture then add it back into the pot for an additional 5 minutes
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This can be stored for 3 weeks in the fridge