Preheat the oven to 175℃/350℉ (no fan) with the rack placed in the middle of your oven. Line your baking dish with 1 tbsp butter, all the way to the brim. Sprinkle in 1/4 cup of finely grated parmesan and swirl the cheese all around ensuring it stick to the butter and coats the surface of your baking dish.
Bring a medium saucepan to a medium heat, melt the butter and add the flour, whisk until well combined and bubbling, cook for about 1 minute.
Gradually pour in the milk whilst whisking, continue whisking until the lumps have dissolved and the milk started to bubble. Turn off the heat and add the salt, pepper, nutmeg and mustard of your choice. Whisk until well combined.
Add the chopped chives and egg yolks, whisk until combined. Switch to a silicon spatula and mix in the grated Gruyère or White Cheddar cheese and remining 1/4 cup parmesan cheese. Mix until mostly melted. Allow the mixture to continue cooling down.
In a large bowl add cream of tartar and egg whites together and whisk on a low speed, gradually increasing the speed until stiff peaks form, about 3 minutes.
Gradually fold in the lukewarm cheese mixture into the egg whites, gently folding until fully combined. Pour the mixture into your butter and cheese coated dish, smoothing out the top. Bake for 35 minutes or until puffy and golden brown on top. Note: It's better to let the souffle bake until properly set and golden but still has a slight jiggle, so try and push it for as long as possible before becoming too dark in colour. This'll ensure a longer hold.
Remove from the oven and serve immediately. Serve with a fresh salad of choice, or with other savoury breakfast items or even with stews - geen skaam!