1. Melt the butter in a medium skillet. Add the onions and sauté until soft, about 2 minutes. Add mushroom and tarragon; continue cooking for 4 to 5 minutes. Add the wine and lemon juice, reduce the heat and simmer until the liquid reduces by half, about 4 minutes.
2. Dissolve the cornstarch in 2 tablespoons of warm water before adding it to the cream. Pour the cream into the skillet and bring to a boil, then reduce to a simmer and cook until it reduces by half, about 5 minutes.
3. Add the cheese and season with salt and pepper to taste. Stir until cheese melts and then set aside.
4. Cut the top of the brioche and pull out the soft center. Arrange them on serving plates and spoon the mushroom filling inside. Replace the top of the brioche and serve immediately.