Remove the stems from the mushrooms and chop them finely.
Fry the bacon in an 8 inch glass baking dish on the highest setting for 3 minutes, until the fat is rendered.
Remove and crumble the bacon.
Add the butter to the bacon fat and brown the mushroom caps for 1 1/2 minutes.
Remove the mushroom caps and fry the chopped stems, scallions, and garlic for 1 1/2 minutes.
Stir in the flour and cook 30 seconds.
Stir in the milk and cook 2 minutes.
Stir in the grated Swiss cheese.
Cook 50 seconds.
Return the mushroom caps and crumbled bacon to the baking dish.
Top with bread crumbs and grated Parmesan cheese.
Place under a preheated broiler for 3 minutes.
The crumbs will not brown in the microwave oven unless you have one of the newest models with the browning attachment.