1. Lightly coat interior of a 1 3/4-quart souffle or other straight-sided baking dish with oil. 2. In a 2- to 3-quart pan, stir milk smoothly into cornstarch. Add marjoram, onion, pepper, and nutmeg. Stir over high heat until mixture comes to a boil. Smoothly puree in a blender with spinach, 2 tablespoons Parmesan, and egg yolks. 3. In a large bowl, beat egg whites and cream of tartar on high speed until whites hold soft peaks. Fold spinach mixture into whites. Scrape into oiled dish. Sprinkle with remaining Parmesan cheese. 4. With a knife tip, draw a circle on souffle top 1" from edge. Bake in a 375F. oven until richly browned and center jiggles only slightly when gently shaken, about 25 minutes.