Preheat your oven to 375°F / 190°C. Generously butter a 2-quart soufflé dish. For an extra-high rise, create a 'collar' by folding a long piece of parchment paper in half, buttering one side, and tying it securely around the outside of the dish, buttered-side in.
In a heavy-bottomed saucepan over medium heat, melt the 4 Tbs of butter.
Whisk in the flour to form a paste (a roux) and cook for 1-2 minutes, stirring constantly, until it smells nutty.
Gradually pour in the hot milk while whisking vigorously to prevent lumps.
Continue to cook, stirring, until the béchamel sauce thickens and is perfectly smooth.
Remove from the heat.
Whisk the egg yolks into the warm sauce one at a time, ensuring each is fully incorporated before adding the next.
Stir in the chopped broccoli, walnuts, minced onion, and Parmesan cheese.
Season generously with salt and pepper.
In a separate, scrupulously clean large bowl, combine the egg whites and cream of tartar.
Using an electric mixer or a clean whisk, beat the whites until they form stiff, glossy peaks (when you lift the beater, the peak should hold its shape).
Gently stir about one-quarter of the beaten egg whites into the broccoli base to lighten it.
Add the remaining whites and use a spatula to carefully fold them in, using an upward motion from the bottom of the bowl. Be gentle to avoid deflating the air you just whipped in.
Carefully pour the mixture into your prepared soufflé dish.
Place it on the middle rack of the preheated oven and bake for 40 to 45 minutes, or until puffed, golden brown, and set in the center.
Do not open the oven door for the first 30 minutes.
Serve the soufflé immediately, right out of the oven, with a dollop of Sour Cream and Horseradish Sauce.
