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Add 2 tablespoons butter and olive oil to a pan and heat over medium heat. Add sliced onions and mushrooms to the pan and sauté, stirring from time to time for about 10 minutes. Season with salt and pepper and add chopped thryme to the pan. Mix and set aside. Transfer filling to a plate.
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Wipe with paper towel the pan and add remaining 2 tablespoons butter to heat over medium heat. Add 2 slices of bread and a bit of cheese to the top of each slice and cook. Once it starts to melt a little add 2 tablespoons of the caramelized onions& mushrooms filling over the top of one of the slices.
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When the bread is golden brown and crunchy, after 2-3 minutes of cooking, sandwich the pieces of bread together and cook on low heat while pressing with cooking spatula until cheese is fully melted.
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Transfer to a plate and repeat with the rest of the ingredients.