Preheat oven to 450 degrees F. Pulse walnuts, parsley and parmesan in a food processor until they look like bread crumbs.
Melt the butter in a Dutch oven over medium-high heat. Add the leeks, and squash and cook, covered, stirring occasionally, until just tender, about 5 minutes. Stir in flour, sage, and nutmeg. Reduce heat add gradually add broth and milk. Simmer, stirring, until thickened, 5 minutes. Season with salt and pepper, and remove from heat.
Cook pasta according to package instructions; drain. Combine pasta with the vegetables and mozzarella. Pour into a 9 by 13-inch casserole. Sprinkle with the walnut topping. Bake until the top is golden, about 25 minutes. Rest 5 minutes before serving.
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