Melt butter in a heavy sauce pot over medium heat until foam subsides. Add the onions, bouquet garni and black peppercorns.
Season onions with 2 tablespoons of kosher salt and increase heat to medium high while stirring vigilantly. When sugars begin to stubbornly adhere to bottom of pan, occasionally deglaze with Chardonnay and Madeira, paying close attention as the additional sugar will accelerate the caramelization.
When onions have reached a uniformly rich brown, add stock. Simmer for 20 minutes. Adjust acidity with sherry vinegar and season to taste. Remove pot from heat and add Cognac to finish.
To serve, finish with a grilled crouton of sourdough and the triple cream cheese Mt. Tam from one of Northern California’s best cheesemakers, Cowgirl Creamery.