Melt margarine, stir in flour & salt; remove from heat. Gradually stir in milk, keeping smooth. Return to moderate heat, stirring constantly. Cook until sauce thickens and comes to boil. Remove from heat. Add onions and cheese. With whisk beat in yolks, one at a time. Stir in tuna, parsley and mushrooms. Cool 10 minutes. Beat egg whites until they hold stiff peaks. Fold into tuna mixture. Turn into ungreased 2 litre souffle dish or large Corningware casserole. Bake in preheated 350 oven until top is golden and knife comes out clean, 45-50 minutes.