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Heat 1/2 the butter and olive oil in a pan.
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Add chopped onion and garlic and heat until soft, about 5-7 minutes. Remove and add to crock pot.
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Heat the other 1/2 of butter and oil oil in same pan.
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Add chopped mushrooms and cook until soft, about 10 minutes. Add garlic powder, thyme, truffle salt (if using) and pepper and stir well. Remove and add to crock pot.
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Add the broth and milk to the crock pot, and cook on low for about 4 hours.
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20 minutes before you are ready to eat, remove about 2 cups of the liquid from the crockpot and pour into a small sauce pan. Add in the flour and whisk well until there are no more lumps. Bring to a boil and let thicken. Add back to the crockpot, stir and change the crockpot setting to "keep warm" for 20 minutes.
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Serve with crackers, croutons, fresh parsley or a drizzle of truffle oil.