INGREDIENTS
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2 tablespoons extra-virgin olive oil, plus more for brushing
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1 small onion, chopped
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1/2 pound white mushrooms, sliced
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Kosher salt and freshly ground pepper
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6 cups baby spinach (about 6 ounces)
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6 slices potato bread, lightly toasted
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6 large eggs
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1/2 cup whole milk
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3/4 cup shredded gruyere cheese