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½ cup butter, at room temperature |
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2 medium size onions, chopped |
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2 lbs (1 k) portobellos, without stems, halved and sliced |
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½ cup fresh parsley, finely chopped |
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¾ cup goat cheese, crumbled |
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¾ cup grated parmesan cheese |
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Grated parmesan cheese, for serving |