In a large resealable plastic bag, combine the first 4 ingredients, shaking well to mix. Add steak and refrigerate for 15 minutes, or up to overnight.
When ready to grill, coat grill rack with cooking spray. Prepare the grill to medium-high heat.
Discard the marinade and grill steak for about 6 minutes on each side, or until desired doneness.
Remove steak to cutting board, cover loosely with foil, and let rest about 10 minutes. Slice steak diagonally across the grain before serving.
While steak is resting, in a non-stick skillet over medium-high heat, heat the butter and oil until butter is melted.
Add mushrooms, stirring until mushrooms are well-coated and cook for 5 minutes.
Add sherry, thyme, salt, and pepper, stirring well to mix. Cook about 5 minutes, until sherry is mostly evaporated.
Serve sliced steak with a portion of the sherried mushrooms spooned over the slices.