1. In a skillet, melt 2 tablespoons of butter. Add the whole mushrooms and brown on both sides. Remove and reserve on warm plates.
2. In the same skillet, heat the remaining butter. Add the shallots; cook for 2 minutes.
3. Add the shrimp and clams; cook for 3 minutes. Deglaze with white wine and, over medium heat, reduce the liquid by a third.
4. Pour in the cream, mix and reduce by half.
5. Transfer this mixture over the portobello and serve.