Sauté the aromatics: Melt the butter and sunflower oil in a large pot over medium heat. Add the chopped onions and white pepper. Cook until the onions are soft and translucent.

Sear the mushrooms: Increase the heat to high. Add the sliced mushrooms and cook until their liquid has almost completely evaporated.

Thicken and deglaze: Stir in the thyme. Sprinkle the flour over the mixture and stir constantly for 30 seconds to cook out the raw flour taste. Whisk in the water and chicken bouillon cubes.

Simmer: Bring the mixture to a boil while stirring constantly. Cover the pot, reduce the heat to low, and let it simmer for 25 minutes.

Blend and strain: Remove from the heat. Puree the soup using an immersion blender. Press the pureed soup through a fine sieve or chinois for an extra-creamy texture.

Finish with cream: Taste and adjust the salt. Return the soup to low heat and stir in the heavy cream. Cook just until heated through, without letting it boil.

Garnish and serve: Ladle the soup into serving bowls. Top with a few fresh mushroom slices, a drizzle of truffle oil, and a pinch of freshly cracked black pepper. Serve immediately.