in a bowl, combine butter, portabella mushroom, garlic, Italian parsley, tarragon until evenly combined.
place compound butter on a sheet of saran wrap, then roll into a tube, tying off on both ends. Place in the refrigerator for at least an hour. Can be held for 1 week.
Season your steaks liberally with salt and pepper, then grill (l or in a skillet on medium high heat with a little butter and oil). about 5-7 minutes per side, until internal temperature is 135-40 for medium-rare/medium.
While your steak is resting, slice up your compound butter and let it melt over the steaks and serve.