Butter one side of each slice of bread then remove the crusts and cut them into 1 ½-inch squares. Depending on the size of your bread, you could be cutting the slices into quarters or sixths. Lay the cut bread buttered side up, in an even layer in a medium-sized baking dish.

In a food processor or with a hand grater, shred the fontina and gruyere cheese, and combine in a medium bowl, then set aside. Shred or grate the parmesan and place in a separate small bowl and set aside.

Beat the eggs, milk, dry mustard, and salt together in a large bowl or in a blender.

Pour the liquid mixture over the bread in the pan. Evenly layer half the cheese blend, half the parmesan, then the remaining cheese blend, and the remaining parmesan. Using your hands, gently press down on the cheese so that the bread is completely submerged under the milk and the milk starts to come up slightly. Cover with plastic wrap and refrigerate overnight.

When you are ready to bake, remove the pan from the refrigerator and preheat the oven to 375F. The pan should rest outside the fridge for 30-45 minutes, you do not want to bake right out of the fridge. Bake the soufflé uncovered until golden brown for approximately 1 hour, turning the pan at 30 minutes. Check to see if the soufflé is fully cooked at 45 minutes and, if needed, tent it loosely with foil and cook for an additional 15 minutes. Remove from the pan from the oven and serve immediately while it’s still warm and puffy.

Optionally, sprinkle with chopped chives for garnish.