3 tablespoons unsalted butter, plus extra for greasing ramekins
3 tablespoons plain flour
1 cup whole milk
salt, to taste
pepper, to taste
½ teaspoon Dijon mustard
4 egg yolks
4 egg whites, cold
½ teaspoon cream of tartar
2 ounces Gruyère cheese, finely grated
1 ounce cheddar cheese, finely grated
Directions
Preheat the oven to 375 F and liberally grease 6 ramekins using unsalted butter.
In a small to medium-sized saucepan, melt the butter. Once the butter is fully melted, add the plain flour to the pan and whisk the flour into the butter until it is completely combined and has formed a paste.
Pour the milk slowly into the saucepan, making sure to whisk fast to incorporate the milk. When the milk is added the roux will quickly thicken, so it is important to keep the paste moving and to keep adding milk so that no lumps form. Keep whisking like this until all of the milk is incorporated and the roux has a silky-smooth consistency.
Remove the roux from the pan and put it into a clean bowl. Allow the roux to cool for a few minutes before mixing in the Dijon mustard and seasoning with salt and pepper.
Mix the egg yolks into the roux one at a time, making sure to thoroughly mix each addition in before adding the next, then set this bowl aside.
Place the egg whites into a large bowl and add the cream of tartar. Using a balloon whisk, beat the egg whites by hand until stiff peaks form. This should only take 3 to 4 minutes. If possible, do not use an electric whisk for this. To make the cheese soufflé rise properly requires the egg whites to be beaten to the point where they stand up in stiff peaks that do not droop, but are not over-beaten. It is almost impossible to judge this correctly using an electric mixer.
Once the egg whites are whisked, gently fold ⅓ of the egg white mixture into the roux. Then, add the Gruyere and the cheddar cheese and fold them in until combined, before folding in the rest of the egg whites.
Transfer the cheese soufflé batter into the prepared ramekins and place them on the lowest shelf of the oven for 30 to 35 minutes. When they have risen and are beautifully golden on top, remove them from the oven and serve immediately.