Preheat grill or preheat oven to 400 degrees. Cut steak into 2-inch chunks and place in a large bowl along with halved mushrooms. 

Stir together melted butter, basil, parsley, thyme, and garlic. Pour over steak and mushrooms and stir to coat. 

Divide steak and mushrooms between four large pieces of aluminum foil (see note 1) and wrap, pinching edges together to seal the packet. Grill for 12-15 minutes flipping halfway through to cook evenly OR bake for 25-30 minutes until steak is cooked to desired doneness (see note 2).

Open packets, garnish with chopped parsley or thyme, and serve immediately.