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Heat a large, heavy bottomed sauté pan over medium heat. Add the butter. As it begins to melt, start swirling the pan frequently and watching the butter carefully. You will notice the butter will get foamy, and then the milk solids will begin to brown. Once that starts, remove skillet from heat. Smell the butter; it should have a nutty aroma, and be caramel in color.
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Add the shallots and rutabaga. Return the pan to medium heat and cook, stirring often, until the vegetables begin to soften, about 10 minutes. Season with salt. Add stock and thyme leaves. Cover, and bring to a boil. Reduce heat to low, and simmer until tender, 20 to 25 minutes. Remove from heat and allow to cool slightly.
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Purée vegetable mixture with mascarpone in a food processor until smooth. Season with pepper.
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Reheat if necessary