Preheat your oven to 400°F (200°C). Place six 8-ounce (235mL) soufflé molds, or one large (1½ quart or 1400mL) mold, atop a lined sheet pan.
Step 2
In a heavy saucepan, melt the 4 tablespoons butter over medium heat. When melted, whisk in the flour to create a roux. Season the roux by whisking in the Dijon mustard, salt, pepper, nutmeg, and cayenne. Continue to cook for another minute, whisking continuously.
Gradually whisk in the milk to create a thickened sauce.
Remove from the heat and stir in the remaining Parmigiano-Reggiano cheese, along with the Gruyère and Comté cheeses and the chives. This is your soufflé base.
Step 3
Add your egg yolks to a medium bowl, and have a whisk at the ready.Add a large spoonful of the soufflé base to the egg yolks, whisking continually, to temper.
Whisk in about ½ cup (120mL) more base to the egg yolk mixture. Return this tempered mixture to the saucepan and whisk until fully incorporated. Set this mixture in the refrigerator to chill.
Step 4
Add the egg whites to a large, clean mixing bowl and have a hand mixer ready, or add to the bowl of your stand mixer fitted with the balloon whisk attachment.Add the cream of tartar and beat the egg whites to stiff peaks.
Step 5
Remove the soufflé base from the refrigerator and stir to loosen. Using a rubber spatula, gently fold one-third of the egg whites into the soufflé base to lighten.
Continue to fold in the remaining egg whites until no streaks remain. If the mixture is not well-combined, your soufflé may rise unevenly or crack.
Step 6
Scrape the mixture into the prepared molds. The mixture should reach the bottom part of the rim.Run your finger along the edge of the soufflés to create a small gap between the mixture and the edge of the mold, which will help the soufflés to rise well.
Step 7
Place the soufflés in the middle rack of the oven, shut the door, and immediately turn the temperature down to 375°F (190°C). Do not open the oven during the cooking process. Bake for 20-25 minutes (less for small, individual molds; more for one large soufflé).
Step 8
When the soufflé has risen well above the mold of the rim, you may test it by slightly jiggling the sheet pan. The soufflés should have a very slight wobble. If you poke with the needle of a kitchen thermometer or long wooden tester, it should come out slightly wet, but not dripping with batter. If it comes out quite wet, continue to bake for another 2-3 minutes.
Step 9
Bring the soufflés to the serving table immediately once they have been removed from the oven.