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Preheat the oven to 350°F. Line a baking sheet with parchment paper.
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Cut the bread into small cubes. Scatter on the baking sheet and bake for 10-15 minutes until toasted golden.
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Heat the butter in a skillet. Add the onion pieces and sauté until golden.
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Stir in the garlic. Sauté until the garlic releases its aroma.
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Add the bread cubes. Toss to coat.
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Pour 1 cup of the reserved liquid over the top.
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Cook, stirring for a few minutes until the bread softens.
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Season with salt and pepper.
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Mix in 1+1/2 cups of the grated mozzarella.
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When the cheese melts, Stir in the chopped parsley.
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Remove from heat.