Heat oil in Dutch oven over medium-high heat. Cook mushrooms in oil about 3 minutes, stirring occasionally, until crisp-tender. Stir in flour, basil, salt, nutmeg and pepper. Cook over medium heat, stirring constantly, until mixture is smooth
and bubbly; remove from heat. Gradually stir in milk. Heat to boiling, stirring constantly. Boil and stir 1 minute.