In a medium bowl stir together all ingredients for the marinade. Add beef chunks, toss to coat, cover tightly and chill for 15 minutes, or up to one hour if you have the time.
Skewer the beef and mushrooms making sure to leave a little space between each piece for even cooking. Discard marinade.
Use tongs to transfer skewers to a preheated grill. Rotate skewers every few minutes until mushrooms are tender and steak is cooked to your desired doneness (for me this was about 8 minutes for medium doneness).
Remove skewers from the grill. Stir together butter and parsley and brush onto the mushrooms just before serving.