Heat olive oil in a large skillet over medium-high heat. Add the finely chopped mushrooms and cook for 8-10 minutes, stirring occasionally, until they’re golden brown and caramelized. Don’t rush this step – proper caramelization is crucial for flavor development.

Add the minced shallots and garlic to the mushrooms. Cook for 2-3 minutes until fragrant and the shallots become translucent.

Pour in the sherry vinegar, Madeira, and port wine. Let the mixture simmer and reduce for 5-7 minutes, stirring occasionally, until most of the liquid has evaporated and the mixture becomes jammy. Season with salt and pepper.

Remove from heat and let the mushroom mixture cool completely to room temperature, about 15 minutes.

While the mushroom mixture cools, heat vegetable oil in a small saucepan over medium heat. Fry the thinly sliced shallot until golden and crispy, about 2-3 minutes. Drain on paper towels and season with salt.

In a mixing bowl, whisk together the mayonnaise and black truffle oil. Fold in the cooled mushroom mixture until well combined. Taste and adjust seasoning as needed.

Transfer the sauce to a container and top with the crispy fried shallots just before serving.