Prick the potatoes with a fork and microwave on high until tender, 10 to 20 minutes.
Meanwhile, heat grill to medium-high. Place the mushrooms and garlic on a large piece of heavy-duty foil and sprinkle with 1/4 tsp each salt and pepper. Fold up the sides and add the wine, then seal the foil to form a pouch.
Season the steak with 1/2 tsp each salt and pepper. Grill the steak to desired doneness, 5 to 7 minutes per side for medium-rare. Transfer to a cutting board and let rest for 5 minutes before slicing.
While the steak is cooking, place the mushroom pouch and potatoes on the grill. Grill the potatoes until just tender, about 5 minutes. Grill the mushrooms, shaking the pouch occasionally, for 8 minutes. Transfer the mushrooms (and any juices) to the bowl and toss with the spinach, parsley and sour cream.
Serve the steak with the mushrooms and potatoes topped with sour cream, if desired.
