Cook the pasta in a large pot of boiling salted water until al dente. Drain very well. Meanwhile, in a medium saucepan, melt the remaining 3 tablespoons of butter. Add the flour and cook over moderately high heat, stirring, until foamy, about 2 minutes. Gradually whisk in the milk and bring to a boil; cook, whisking until thickened, about 4 minutes. Add the cream and nutmeg and season with salt and pepper. Transfer the sauce to the bowl with the radicchio.