First, prepare your baking vessel. Generously butter a 2-quart soufflé dish and coat the inside evenly with the grated Parmesan cheese. For an extra-tall rise, create a collar by buttering a long piece of parchment paper or aluminum foil and tying it securely around the outside of the dish, extending at least 2 inches above the rim.
Set aside.
Preheat your oven to 400°F / 200°C.
Place the dried mushrooms in a bowl and cover with 1½ cups of boiling water.
Let them soak for at least 2 hours, or overnight, until fully rehydrated.
Drain the mushrooms, reserving the flavorful soaking liquid. Carefully rinse the mushrooms to remove any grit, then chop them coarsely.
Strain the reserved mushroom liquid through a fine-mesh sieve or cheesecloth to filter out any remaining grit. You will need 1 cup (240 ml) of this liquid.
In a saucepan over medium-low heat, melt 3½ tablespoons of the butter.
Whisk in the flour to form a paste (a roux) and cook for 2 minutes, stirring constantly.
In a separate bowl, whisk the hot milk and sour cream together, then gradually pour this mixture into the roux, whisking continuously until the sauce is very thick and smooth.
Remove the sauce from the heat.
Season with salt, pepper, and a pinch of cayenne.
Beat in the egg yolks one at a time, ensuring each is fully incorporated before adding the next.
Stir in the chopped fresh dill and set the sauce base aside.
In a medium skillet, melt the remaining 2 tablespoons (30 ml / 1 fl oz) of butter over medium heat.
Add the chopped onion and shallots and sauté until soft and golden.
Add the chopped mushrooms, season with a little more salt and pepper, and cook for 2-3 minutes.
Pour in 1 cup (240 ml) of the reserved mushroom liquid.
Continue to cook, stirring occasionally, until all the liquid has been absorbed.
Stir the hot mushroom mixture directly into the sauce base, mixing until everything is well combined.
In a large, impeccably clean bowl, beat the egg whites with a pinch of salt and the cream of tartar using an electric mixer. Start on low speed and gradually increase to high, beating until they form stiff, glossy peaks.
Gently lighten the mushroom base by stirring in about a quarter of the beaten egg whites.
Then, pour the lightened base over the remaining egg whites.
Using a large spatula, carefully fold the mixture together until just combined, being careful not to deflate the whites.
Pour the soufflé mixture into your prepared dish.
Place it on the middle rack of the preheated oven and immediately reduce the temperature to 375°F / 190°C.
Bake for 35 to 40 minutes, or until the soufflé is puffed, beautifully golden-brown, and a skewer inserted into the center comes out mostly clean.
Serve immediately with a drizzle of Hollandaise or Spicy Mousseline Sauce.