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In a large stock pot, heat butter on medium high heat. Add in both onions and garlic cloves. Simmer until the onions start to soften, about 3 minutes.
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Add in thyme, salt, pepper, and balsamic vinegar. Mix together. Continue sauteing until onions start browning and getting a bit sticky.
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Add in beef broth & Worcestershire. Stir and bring to a boil. Reduce to a low simmer and continue cooking for another 5 minutes.
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Meanwhile, preheat the broiler. Set two ramekins or oven safe bowls in a baking dish. When soup is done, ladle into bowls, leaving ½ ” at the top. Put 2-3 slices of the baguette in each bowl. Top each with Swiss cheese and place under the broiler for 3-5 minutes or until cheese is bubbling.
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Let cool a bit & enjoy!