Finely chop the mushroom stems. Stuff each mushroom with a brie cube.
In a pan, add the oil and leeks and cook for 4 to 5 minutes over medium heat until softened. Add the pecans, cumin, sage, and tarragon, and cook for 2 to 3 minutes until fragrant. Add the mushrooms and cook until reduced in volume and darker in color, about 3 minutes, adding more oil as needed.
Add the garlic and breadcrumbs and cook for 1 to 2 minutes until toasty. Add the vegetable stock and cook until the mixture turns into a paste, 2 to 3 minutes. Season with salt.
Fill each mushroom with about a teaspoon or two of the filling, scraping the filling against the sides to form a dome.
Heat the oven to 425°F.
Roughly clean the pan with a paper towel. Add more oil and sear the stuffed mushrooms in the same pan for 1 minute. Transfer the pan to the oven and cook for 10 to 15 minutes, until the mushroom mixture looks crispy on top.
For the sauce, simmer the wine until reduced by half. Add a tablespoon of butter at a time while constantly whisking, then repeat with the remaining butter. Season with salt and set aside.
Season the mushrooms with flaky salt and chives on top, and serve warm with a little wine sauce drizzled on each.
