Prep work
Rinse spinach thoroughly.
Cook spinach (rinse then cook 3 to 5 minutes until tender). Drain then chop the spinach.
Heat oven to 350 degrees. Butter 4-cup soufflé dish or 1-quart casserole.
Step 2
Heart butter in saucepan over low heat until melted. Blend in flour, 1/4 t. salt and pepper. Cook over low heat, stirring constantly until mixture is smooth and bubbly. This is your white sauce.
Remove from heat.
Step 3
Stir in milk. Heat to boiling, stirring constantly. Boil and stir for 1 minute, then remove from heat.
Stir in onion, 1 t. salt and the nutmeg.
Step 4
Beat egg whites and cream of tartar in large mixer bowl until stiff. Beat egg yolks in small mixer bowl until very thick and lemon colored. Stir into white sauce mixture. Stir in spinach.
Stir about 1/4 of the egg whites into sauce mixture -- fold into remaining egg whites.
Step 5
Carefully pour into soufflé dish. Set soufflé dish in pan of water (1 inch deep). Bake until puffed and golden and until knife inserted halfway between center and edge comes out clean -- 50 to 60 minutes.