Step 1 Heat a wide, heavy-bottomed stock pot or Dutch oven over high heat until hot. Add the vegetable oil, then half the mushrooms. The mushrooms will immediately begin to sizzle. Cook quickly, stirring constantly, until the mushrooms begin to brown, 4 to 6 minutes. The mushrooms should cook quickly enough that they brown before they give up any moisture; this will give them a nice nutty flavor. Remove from heat and spread the mushrooms onto a plate to cool. Cook the remaining mushrooms the same way, adding a little additional oil if needed. Remove from heat and add to the first batch of mushrooms, seasoning with one-half teaspoon salt and one-fourth teaspoon pepper.
Step 2 In the same pot, heat the olive oil over medium-high heat until hot. Add the onion and cook, stirring frequently, until translucent and browned slightly, 8 to 10 minutes. Stir in the garlic and cook until aromatic, an additional minute or so. Add the wine and cook, scraping any flavoring from the base of the pan.
Step 3Add 3 cups vegetable broth and half the mushrooms. Once the soup comes to a simmer, remove from heat and purée using an immersion or stand blender. Return the soup to the pot and whisk in the macadamia or cashew cream. Stir in the remaining mushrooms and bring to a bare simmer, stirring frequently. Taste, and add three-fourths teaspoon each salt and pepper, or as desired.
Step 4Continue to cook for 10 to 15 minutes to marry the flavors. The soup will thicken as it cooks; adjust the consistency with additional broth or water as desired. This makes a scant 2 quarts soup.
Step 5Taste, and adjust the seasonings again if needed. Serve the bisque garnished with chopped chives and a drizzle of truffle oil.