Roasted Flank Steak with Mushrooms and Thyme

flank steak, about 1 1/2 lb. trimmed

chopped fresh thyme

dry red wine

garlic cloves, peeled, sliced

salt and ground pepper

olive oil for marinade plus 1 Tbsp for a cooking pan

onion, peeled and cut into 6 wedges

unsalted butter

mushrooms, sliced

salt and freshly ground pepper,