Melt 2 Tablespoons butter over low heat in a large saucepan. Add celery, onions and red pepper flakes and simmer slowly the ingredients over low heat for 20 minutes. The ingredients should not turn brown, but you want them to cook slowly until the celery softens and the onions begin to turn transparent. Add cream, mushrooms, shrimp, crab, and salt to the pan. Turn the heat up a little bit until the liquid begins to bubble, then bring the heat back down and let simmer for 20 minutes or until it reduces to about 1/2 the volume and becomes thick. Watch the saucepan carefully to make sure the mixture doesn’t bubble over.