Preheat the oven to 400 degrees F and place a rack in the center of the oven.
Stir the cheese soup over low heat, adding the mustard, and stirring in the egg yolks one by one.
Beat the whites and fold them in gently.
Spoon into a buttered souffle dish, set in the oven, reduce the heat to 375 degrees F, and bake for 30 minutes.
several recipes have told me not to butter the dish as it wont climb properly and thus wont rise, will have to try it with and without buttering. let me know which works better if you try it. i don't keep dry mustard in the house generally so i might try using other spices like dill, garlic, and onion salt. bit of chipotle or other spice would be nice and i'm picturing this with a sprinkling of paprika for garnish. wonder if you could do this with other soups? mushroom or cream of broccoli maybe, hmmmm. adding spinach or mushrooms to the soup part would be nice too.