In a small bowl mix together the marinade ingredients.
Wash and scrub potatoes clean; dry and cut larger potatoes in half. Clean mushrooms with a paper towel and remove stems.
Place the potatoes and mushrooms in one zip top bag. Place the cubed steak into a separate zip top bag. Pour half of the marinade into each of the two bags and zip close.
Shake each bag to evenly coat the steak, potatoes and mushrooms. Place in the refrigerator and marinate for at least two hours, or overnight. (note: Prep time listed above does not include marinating time)
Preheat the Indoor/Outdoor grill on Setting 4.
To assemble the kabobs, skewer 2 pieces of steak and one mushroom, repeating the process so there are three mushrooms and 8 pieces of steak on one 10" skewer. Repeat for the remaining three skewers.
For the potatoes, tear off a large piece of aluminum foil, folding up all four sides to create a "basket" for the potatoes. Place 2 pats of butter on top of the potatoes and cover with another piece of foil.
Place all four kabobs on one side of the preheated grill and potatoes on the other side. Place the lid on the grill. Grill the kabobs for 3-4 minutes per side for medium rare, and 5-6 minutes per side for medium.
Continue to grill the potatoes for 35-40 minutes, or until fork tender.