Combine the oil, garlic, rosemary, and salt and pepper, to taste, in a medium bowl. Add the steak and coat well. Allow to the steak to marinate for 20 to 30 minutes while preheating the grill. If using a gas grill, preheat the grill to high, direct heat. If using coals, allow the coals to burn down to glowing white embers. The grill is hot enough when you can only hold your hand about 1-inch over it for only a second.
Lightly grease the grill with olive oil to prevent sticking. Grill the steak for 4 to 6 minutes per side, or to desired doneness. Remove the steak from the grill to a cutting board. Allow it to rest for 10 minutes, tented in foil to maintain the heat.
Meanwhile, in a large pan over medium-high heat, add the butter and when melted saute the garlic until it softens, about 1 minute. Don't allow it to brown. Add the spinach and saute until dry. Add the cream, Parmesan and nutmeg and cook until thick and creamy. Season with salt and pepper, to taste. Transfer to a serving bowl and set aside until ready to serve.
If using wagyu, cut the steak into portions. If using flank steak, carve it into thin slices, against the grain, and at a slight diagonal so that the slices are wide. Arrange the steak on a serving dish and serve warm with the spinach.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.