Place butter in a large, deep saucepan over medium heat.
Add shallot and gently sauté, stirring often, for about 3 minutes.
Add red onion, button mushrooms, carrot and red bell pepper. Continue cooking, stirring occasionally, for about 4 minutes.
Stir in flour and cook until it turns slightly golden, about 5 minutes.
Gradually whisk in stock, stirring constantly, until soup comes to a simmer.
Reduce heat to medium-low, add cream and cubed brie cheese. Stir until cheese melts. Do not return soup to a boil.
Add salt and several grinds of ground black pepper.
If sparkling wine is refrigerator temperature, warm in a microwave (for about a minute) and slowly add to soup (it will foam up).
Heat soup, stirring constantly, until it is hot, but not boiling.
Taste and adjust seasoning. Serve immediately.
