Meanwhile, in the large deep skillet, melt the butter. Add the flour and cook over moderate heat, whisking constantly, until the roux begins to smell toasty, about 2 minutes. Whisk in the half-and-half and simmer over moderately-low heat until the sauce begins to thicken, 3 to 5 minutes. Do not let the half-and-half come to a boil. Whisk in the pumpkin purée, grated parmesan, lemon juice, garlic, and nutmeg until smooth. Season with salt.