Preheat oven to 350F.
Heat oleo until melted. Add flour, ¼ tsp salt, and pepper. Cook until smoothe and bubbly. Remove from heat.
Add milk and heat to boiling. Boil, stirring constantly, for about a minute. Remove from heat.
Add onion, remaining salt, and nutmeg. Beat egg whites and cream of tartar in a 3-quart bowl. Beat egg yolks in a small bowl until very thick. Add yolks to sauce. Add spinach and ¼ of the egg whites. Fold sauce into the remaining egg whites.
Pour mixture into a 1-quart buttered casserole dish. Place casserole dish in a pan of water (about 1-inch deep). Bake for 50 to 60 minutes until puffed and golden.