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Heat olive oil and butter in a large pan over medium heat until butter is melted.
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Add onions, beef broth, Worcestershire sauce, garlic, sugar, thyme, and salt and pepper. Stir to combine.
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Turn up heat to high until broth starts to come to a boil.
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Turn heat back down to medium and cook, stirring occasionally, for 35-40 minutes, or until onions are very soft, dark, and brown. (If it appears that the onions are starting to burn, turn the heat down to medium-low heat.)
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Remove from heat and let cool completely. (To speed it up you can put the onions and juices in a bowl or in the blender jar.)
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Once cooled, placed caramelized onions and juices in a blender (I like to use my Blendtec).
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Puree until smooth.
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Add cream cheese and blend until smooth.
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Add sour cream and blend until well combined.
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Add cayenne pepper to taste and blend to distribute evenly.
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Put dip in serving bowl and sprinkle cayenne on top (if desired).
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Serve with crackers, chips, other bread, or vegetables.