"Frittatas are excellent served warm or at room temperature, which means that this meatless main is a perfect do-ahead dish...."
INGREDIENTS
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2 tablespoons olive oil, divided
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2 medium leeks, whites and pale-green parts only, chopped
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8 ounces crimini (baby bella) mushrooms, thinly sliced
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12 large eggs
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1/2 cup crème fraîche or sour cream
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2 tablespoons coarsely chopped flat-leaf parsley
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3/4 cup shredded Fontina cheese, divided
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Kosher salt, freshly ground pepper