1. Cut off and discard root ends and dark green leaves of leeks. Cut leeks in half lengthwise and swish around in a large bowl of water to rinse; transfer to a colander, leaving dirt behind, and rinse again in fresh water if necessary. Thinly slice crosswise into semicircles.
2. Melt 2 tablespoons butter in a 5-quart saucepan (or Dutch oven) over medium heat. Add leeks, mushrooms, 1 teaspoon salt, and the nutmeg; cook, stirring often until vegetables are soft and beginning to brown, 12 minutes. Using a slotted spoon, transfer leek mixture to a bowl and set aside. Set aside pan with cooking juices in it.
3. Bring a large pot of water to a boil. Add 3 teaspoons salt and the egg noodles and cook until they are barely tender to the bite. Drain noodles and set aside, covered.
4. While noodles are cooking, melt remaining 3 tablespoons butter in same frying pan or saucepan. Sprinkle in flour and cook, stirring until mixture looks glossy and golden brown, about 3 minutes. Whisk in broth and sherry and simmer until thickened, about 3 minutes. Remove from heat and whisk in half-and-half and Parmesan. Add remaining 1 teaspoon salt if needed. Reduce heat to low, return pan to heat and stir in leek mixture, turkey and 1/3 cup parsley.
5. Just before serving, stir in radicchio. Serve over cooked noodles and garnish with more chopped parsley.
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