Halve eggs lengthwise, press yolks through strainer; mix with mushrooms and mayonnaise to moisten.
Season with salt.
Pile mixture back into whites; spread 1 cup Mornay Sauce in flat baking dish.
Place eggs on top of sauce, and spoon remaining sauce over.
Sprinkle with cheese, and heat in 350Ã‚Â°F. oven until sauce bubbles.
Serve at once.