Heat a very large heavy skillet (cast iron works best) over medium-high heat.
Add the butter, olive oil, and onions and sauté 15 minutes, until tender and well caramelized.
Add the garlic and apple slices and cook for another 2 minutes to soften the apples. Add salt and pepper and stir to scrape any brown bits on the bottom. Stir in the wine and continue to cook until the liquid evaporates, then cool. (The onion mixture can also be prepared up to 2 days ahead and stored in an airtight container in the refrigerator).
Preheat the oven to 350°F. Unwrap the Brie and cut off only the top rind of the cheese, leaving sides and bottom intact. Place the Brie, cut side up, in a round ceramic baking dish slightly larger than the Brie round. Top the Brie evenly with the caramelized onion mixuture. Place the dish on a baking pan to catch any cheesy drops. Bake 30 minutes, until melted, oozing and bubbling. Remove from the oven and transfer to a heatproof platter.
Serve with crackers, baguette slices, and crostini.

Contribution by: Simply Delicious Wine Country Recipes by Robin Goldstein