Thinly slice the onion, mushrooms, and garlic. Add to the pan and sauté gently for 10-20 minutes, stirring occasionally, until the mushrooms and onions begin to caramelize.
Add in the marsala wine to deglaze the pan, making sure to scrape up any brown bits off the bottom of the pan. Cook for another 2-3 minutes.
Remove from the heat and set aside. Then season with salt and pepper.
Place the slices of bread on a baking sheet and top with a slice of brie.
Then place in the oven under the broiler for about 5 minutes, or until just starting to toast and the cheese softens. Remove from oven.
Top each slice of bread and cheese with a small spoonful of the caramelized mushrooms and onions.
Sprinkle with fresh thyme and enjoy!
Notes
Crostini are best eaten the same day. However, the mushrooms will keep in the fridge for up to 3 days.