Warm the milk in a saucepan to heat it; do not boil. In another saucepan, melt 3 tablespoons of butter over medium heat. Once melted and foamy, add the flour, dry mustard, garlic powder, cayenne (if using), salt, pepper, and nutmeg. Whisk constantly for 2 minutes to cook the flour and create a smooth liquid. Gradually pour in the warmed milk while whisking constantly to prevent lumps. Continue whisking as the mixture heats. It will start to thicken considerably. Once it reaches a boil, reduce the heat to low and continue cooking and whisking for 2-3 minutes until very thick – it should coat the back of a spoon heavily. Remove from heat.

Separate the egg whites and yolks. Add the egg yolks to a small bowl and whisk with a fork. To temper the yolks, add a bit of the hot béchamel to the yolks while whisking vigorously. Add another spoonful, and keep whisking. Now pour the tempered yolk mixture back into the saucepan with the remaining béchamel, whisking to combine. Add both the grated cheeses while stirring and keep going until completely melted. The mixture should be thick, smooth, and glossy. Taste and adjust seasoning if needed. Transfer to a large bowl and let cool for 5-10 minutes while you whip the egg whites.

Add the egg whites to a clean, completely grease-free bowl of a stand mixer (or use a hand mixer). Add ¼ teaspoon cream of tartar and beat on medium speed until the whites become foamy, about 1 minute. Increase speed to medium-high and continue beating until the whites form stiff peaks and are cloudy, about 3-4 minutes total. When you lift the whisk, the peaks should stand straight up without drooping.

Take about ⅓ of the whipped egg whites and add them to the cheese mixture. Using a rubber spatula, fold them in vigorously. Add the remaining egg whites and fold very carefully, until the whites are just incorporated – you should see no large white streaks, but a few small streaks are fine. Do not overmix or stir vigorously to avoid deflating.

Take out the prepared soufflé dish from the refrigerator. Carefully pour the soufflé mixture into the dish, filling it to about ¾ inch from the top (or ½ inch for individual ramekins). Use a spatula to smooth the top. Tip: run your thumb around the inside rim of the dish, creating a shallow groove about ½ inch deep all the way around – this helps the soufflé rise evenly and creates that classic ‘top hat’ shape. You can sprinkle a tiny bit more Parmesan on top.

Place the soufflé dish on a baking sheet and transfer to the preheated oven on the lower rack. Bake for 30-35 minutes for a large soufflé (20-25 minutes for individual ramekins). Do NOT open the oven door for at least the first 20 minutes! The temperature drop will cause your soufflé to collapse. After 20 minutes, you can peek through the oven window. The soufflé is done when it has puffed up 2-3 inches above the rim, the top is golden, and it jiggles only very slightly when shaken. For a more set interior, bake an additional 3-5 minutes. The centre should still be slightly creamy.

Remove the soufflé from the oven and serve immediately – soufflés begin to deflate within 2-3 minutes as they cool. Bring it directly to the table while it's still puffed. To serve, use two large spoons to gently puncture the top and scoop portions vertically from the top down – do not dig into the sides or bottom, to avoid its collapse. Each serving should have some of the golden crust and the creamy interior.