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Heat 1 tbsp butter and 1 tbsp olive oil in a sauce pan. Add chopped onion and sauté on low until caramelized, continually stirring, for about 15-20 minutes. Remove onions from pan and set aside.
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Add mushrooms to pan and sauté until soft, about 5 minutes. Remove mushrooms from pan and set aside.
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Add 1 tbsp butter and olive oil to the pan and once melted, add 1 tbsp flour and mix well, making a roux.
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Once light brown, add back the onions, garlic and mushrooms to the pan. Cook for 2-3 minutes, mixing well.
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Add 1/2 cup chicken stock and 2 tbsp champagne wine vinegar and mix well. Bring to a boil and let reduce for 2-3 minutes.
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Add 1/4 cup milk and remove from heat. Add parsley and truffle oil and stir well.
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Once pork loin is done, remove from oven and slice. Serve with mushroom truffle sauce drizzled over top.