In a large Dutch oven, heat butter and olive oil over medium heat.
Sauté the leeks and shallots until tender. Add garlic.
Stir in mushrooms and sauté for another minute.
Add the flour and stir to coat the vegetables.
Pour in the stock and bring to a boil. Continue to stir until the mixture thickens. Lower to a simmer and cook uncovered for 20 minutes.
Add cream, brie, thyme, and nutmeg. Simmer for 5 minutes.
Stir in the sherry and lemon juice.
Season with salt and pepper.