Using a sharp knife, remove the top rind of the cheese.
Place in a small cast iron skillet or baking pan and bake at 350 degrees for 10-15 minutes, or until softened.
While brie is baking, heat olive oil and butter in a small skillet over medium heat until butter is melted.
Add mushrooms, green onion, pepper, and thyme; saute until mushrooms are browned.
Add marsala wine and continue cooking, stirring occasionally, until sauce is thickened.
Remove brie from oven; top with mushrooms.
Return to oven and bake an additional 5 minutes or until brie is hot and bubbly.
Garnish with fresh thyme (optional) and serve with crackers or bread.