Slice Mushrooms or leave whole. Sautee Mushrooms in the butter. Add Group 2 and bring to a rolling boil. Mix Group 3 into a slurry and add to mushroom mixture. Bring to the boil while stirring for 2-3 minutes. Cool filling down then place in refrigerator until required.
For recipe variation: add 500 g chopped sauteed Bacon prior to cooling.