Melt butter in a skillet over medium heat. Toss in sliced onions, whiskey, brown sugar, thyme, and a pinch each of salt and pepper. Reduce heat to medium-low and cook, stirring every few minutes, for at least 20 minutes until deep brown and jammy. Remove from skillet into a bowl or storage container and cover to keep warm.
Preheat oven to 350 degrees.
Place brie wheel, with the rind intact, on a piece of parchment paper. Transfer to a baking sheet and set in the oven for about 15 minutes. Tap the top of the brie without piercing once time elapses. If it is wiggly and pliable, the brie is ready.
Remove from oven and set onto a serving platter of choice. Top with onions and additional fresh thyme, as desired. Serve with choice dipping vessels. I opted for baguette, apples, grapes, and broccoli.