Melt the butter in your saucepan. Stir in the flour to make a paste. Cook it for one minute on low heat.

Gradually pour in the cold milk. Whisk constantly to avoid any lumps. The sauce should become very thick and smooth.

Remove the pan from the stove heat. Stir in the mustard and cheddar cheese. Let the mixture cool down for a bit.

Add the egg yolks to the cheese sauce. Mix them in until the color is even. Put this mixture into a large bowl.

Whisk the egg whites in a separate bowl. They must reach the stiff peak stage. Do not overbeat the egg whites today.

Fold one spoon of whites into the sauce. This lightens the heavy cheese base first. Gently fold in the remaining egg whites.

Pour the mixture into your prepared dish. Run a thumb around the inside rim. This helps the souffle rise straight up.

Bake the souffle for twenty-five minutes. Do not open the oven door early. Serve it immediately while it is still tall.