Spray baking sheet with nonstick spray. Preheat oven to broil.
In mixing bowl, toss together minced mushroom stems and lemon juice.
Heat sauté pan over medium heat. Melt butter and add stems and sauté 5 to 8 minutes, or until mushrooms release their juice and it evaporates. Add shallot and garlic and sauté until soft. Add breadcrumbs, sour cream, nuts, salt, pepper, and herbs. Stir together and sauté until liquid evaporates. Remove from heat. Stir in brie.
Divide the mixture between the mushroom caps. Place under the broiler and broil 3 to 5 minutes until golden brown and cheese is melted and bubbly. Serve hot. Serves 4 to 6 as appetizers.