Clean mushrooms, leaving stems but cutting off any hard stem ends. If mushrooms are larger than 1 inch or so, cut them in half, through the stem (lengthwise).
Melt butter in large saute pan over medium heat. Add mushrooms and sauté about 4-5 minutes, until they start to brown a bit and butter is mostly absorbed into mushrooms. Add the thyme and whole garlic clove and sauté for a minute. Add the marsala, cooking for 1 minute to release alcohol. Add beef broth and dry porcini mushroom, as well as a pinch of salt and pepper. Turn down heat and simmer gently, uncovered, stirring occasionally, for at least 30 minutes, until liquid is reduced by half.
At this point you can remove from heat, set them aside, and let sit in their juices for up to 2 hours.
When getting ready to serve, return to low heat and bring to a simmer, stirring occasionally, until mushrooms absorb most of the liquid and the liquid is reduced to about 1/2 C (just a thin layer of liquid under the mushrooms, but not totally dry). This may be 15-20 minutes. Remove garlic clove, thyme sprig (if using) and the porcini mushroom.
Season with salt and pepper to taste. (Be sure to taste it as it may need more salt than you think). Serve with their juices on top of steak or as a side dish.
Enjoy!