In a large skillet add the beef on medium high heat and cook well, breaking it apart but leaving some chunks about the size of a raspberry or small grape (don't crush the heck out of it, the larger crumbles will help give the sandwich stability).
Cook until well browned (5-6 minutes), then remove from the pan.
Add 1 tablespoon of butter along with the sliced mushrooms and cook for 2-3 minutes on medium high heat until they're browned, then remove them from the pan.
Add in the remaining butter, diced mushrooms, onions and garlic to the pan and cook for 2-3 minutes on medium high heat until they're browned.
Continue cooking until the sauce starts to evaporate and the mixture becomes "sloppy" (about 5-6 minutes).